An egocentric straight-ahead bias in primate’s vision

We saw in the introduction that modulations of neural excitability triggered by changes in gaze direction are ubiquitous in the primate visual cortex and can be observed as early as primary area V1. The demonstration was notably brought by Trotter and Celebrini (1999), who performed extracellular single-unit recordings in the dorsal part of V1 in behaving macaque monkeys trained to maintain fixation at various gaze angles. After isolating the spiking activity of single neurons under the tip of the microelectrode, their receptive fields (RF) were characterized and subsequently stimulated with luminance square-wave gratings and dynamic random dot stereograms. Neural responses to the same visual stimulations were recorded, while the animals were instructed to gaze in different directions. In most recorded neurons (~ 70%), the spiking activity was found to vary significantly as a function of gaze direction. Importantly, changes in the direction of gaze mostly affected the overall level of the neuron’s excitability (i.e., their gain), but only seldomly their tuning for orientation and binocular disparity.

Neurons in the dorsal part of area V1 have small RFs subtending a few degrees around the fixation target (retinal eccentricity less than 5 degrees). They are in charge of central vision. Some years later, the same team (Durand et al. 2010) used a similar approach to investigate the effects of gaze direction on V1 neurons lying within the calcarine sulcus and in charge of the contralateral peripheral visual field (retinal eccentricity between 7 and 35 degrees). Gain modulations associated with the direction of gaze were found to be as frequent in peripheral V1 as in central V1 (~ 70% of the recorded neurons). However, a notable difference was that most peripheral V1 neurons exhibited their maximal gain for the gaze direction bringing their RF along the straight-ahead direction. To confirm this observation, multi-unit recordings were performed in both central and peripheral V1 of the same animal. Usually, both aspects of area V1 were tested within the same recording sessions, since the penetration angle of the microelectrode was set to encounter first the dorsal part of V1, and then its deeper calcarine sector. Once the population RF of a recording site was localized, 5 directions of gaze bringing the RF between 10° left and 10° right, with 5° steps were successively tested (Fig. 2, upper line). Across the multi-unit recording sites, gain modulations associated with changes in gaze direction were usually weaker in central than in peripheral V1, indicating that gaze direction preferences are probably more balanced across neighbouring central V1 neurons and tend to cancel out when averaged together (Fig. 2, lower-right panel). In peripheral V1, the bias for the straight-ahead direction was fully confirmed, with the population gain profile showing an inverted ‘V’ profile peaking at the straight-ahead direction (Fig. 2, lower-left panel). Further control experiments showed that this effect could not be accounted for by slight gaze-dependent variations in the luminance, position, size or binocular disparity of the visual stimulation. Thus, such a gain profile indicates that at the population level, peripheral V1 neurons are maximally sensitive to visual stimuli located straight-ahead. Although the straight-ahead preference was not observed in the spontaneous activity of V1 neurons, it was already present in the earliest components of their visual responses (at about 40 ms post stimulus onset). A subsequent study reported that a neuronal preference for the straight-ahead direction might also be present across central V1 neurons (Przybyszewski et al. 2014). This apparent discrepancy between the results of Durand et al. (2010) and those of Przybyszewski et al. (2014) concerning central V1 might be due to differences in the protocols used in both studies. The latter used bright or dark bars optimized in terms of orientation, length, velocity and color for the recorded neurons so as to measure their contrast response functions. The gain effect that was reported was significant only for the dark bars and statistically weaker at the population level. It would be worthwhile to confirm the presence of a straight-ahead bias in central V1 through multi-unit recordings. At this stage, one can only postulate that if this bias exists, it is less strong and systematic (in terms of stimuli parameters) than that observed in peripheral V1.

These results are important for two main reasons. First, they show that gaze-dependent gain modulations do not necessarily average out at the population level and thus, that they could have a significant influence on the overall visual sensitivity per se. Second, they reveal that a greater visual sensitivity is allocated to the objects we are facing, which probably reflect their special behavioural status, notably because they represent potential obstacles during locomotion. If this mechanism is present in the visual cortex of macaque monkeys, an important question is whether it can also be found in that of humans. The following section provides evidence gathered through non-invasive macroscopic approaches (fMRI and EEG) that this is actually the case.

In this section, we present two studies that investigated whether the straight-ahead direction is also associated with privileged processing in the human visual system using macroscopic recordings (fMRI and EEG). The first study to test whether the straight-ahead direction modulates responses in the human visual cortex was based on fMRI recordings and conducted by Strappini and collaborators (2015). These authors combined standard phase-encoded stimuli (flickering checkerboards in either a wedge or a ring configuration for polar angle and eccentricity mapping, see, e.g., Sereno et al. 1995) and wide-field stimulation (see Pitzalis et al. 2006) to characterize the retinotopic properties within the visual cortex of 6 participants. By determining the preferred polar angle and eccentricity of each visually responsive voxel from an analysis in the Fourier domain, this approach permitted to delineate the borders of several retinotopic areas (notably V1, V2, V3, V4 and V3A) and also to define subregions of V1 and V2 responding to stimulations along the horizontal or vertical meridians. In the same group of subjects, they subsequently performed recordings to test for the interaction between retinotopy and gaze direction. They used a 10° radius flickering checkerboard wedge rotating in a counter-clockwise direction around an ocular fixation point located either straight-ahead (gaze center condition) or 20° up (gaze-up) or down (gaze-down), along the vertical meridian (see Fig. 3A, upper panel). They found that although gaze direction did not modify the preferred polar angle of the responsive voxels (see supplementary figure 9 in Strappini et al. 2015), it could nevertheless modulate their response gain. This is in agreement with the results of a previous fMRI study that investigated the relationship between gaze direction and retinotopic properties (Merriam et al. 2013). Gain modulations were nearly absent in the subregions of V1 and V2 coding for the horizontal meridian, while they were strong and consistent in subregions coding for the vertical meridian. By examining in more details the response profiles in the dorsal and ventral part of these subregions of V1 and V2, they found that in dorsal V1 (V1d) and dorsal V2 (V2d), response amplitudes to stimulation along the lower vertical meridian were significantly stronger (BOLD responses were up to 25% more important) for the gaze-center and gaze-up conditions when compared to those of the gaze-down condition. At the opposite, in ventral V1 (V1v) and ventral V2 (V2v), response amplitudes to stimulation along the upper vertical meridian were significantly stronger for the gaze-center and gaze-down conditions when compared to those of the gaze-up condition (see Fig. 3A, lower panel for results in V1v and V1d). The lower responses observed in V1 and V2 when stimuli are localized more eccentrically with respect to the eye level demonstrate a preference for straight-ahead stimulation in these two areas.

The privileged processing of the straight-ahead direction that was first demonstrated in macaque V1 using single-cell and multi-unit recordings (Durand et al. 2010, see the previous section) thus generalizes to macroscopic recordings in humans. Interestingly, the strength of the gain modulations reported in humans (i.e., an improvement of the BOLD responses by as much as 25% for straight-ahead stimuli) is quite consistent with the spike-rate increases observed in macaques (i.e., about 20–40% when gaze direction puts the receptive field straight-ahead the head/body midline), which suggests similar processes of the straight-ahead direction in the two primate species. Finally, it seems important to point out that gaze direction in the fMRI study was distributed along the vertical meridian, whereas it was distributed along the horizontal meridian in the electrophysiological study. It suggests that the privileged processing of the straight-ahead direction in the visual system of human and non-human primates is operant along both the horizontal and vertical axes of the egocentric space. This hypothesis is in line with the fact that gain modulations were reported for gaze direction along both the horizontal and vertical direction. For example, in their human fMRI study, Merriam et al. (2013) were able to decode between 8 different eccentric eye positions (either along cardinal directions or on the diagonals) from response amplitudes in several visual areas, and notably V1. Note, however, that this study did not explore whether specific spatial positions (e.g., the straight-ahead direction) were systematically associated with gain increases.

If the spatial resolution of fMRI is good enough to determine that straight-ahead effects occur in early visual areas, and notably in V1 (Strappini et al. 2015), its temporal resolution remains too poor to capture the dynamics of the underlying mechanisms. To characterize these dynamics, Bogdanova et al. (2020) performed EEG recordings in human participants. Their stimulus was a flickering checkerboard wedge at high contrast that was presented along the horizontal meridian (i.e., at eye level) at 10° of eccentricity either to the left or to the right of ocular fixation. As shown in Fig. 3B (upper panel), gaze direction was also manipulated along the horizontal meridian so that during leftward gaze (− 10°), visual stimulations in the right visual field were straight-ahead the head/body midline, whereas visual stimulations in the left visual field were eccentric (i.e., at − 20° with respect to the straight-ahead direction). At the opposite, during rightward gaze (+ 10°), visual stimulations in the left visual field were straight-ahead, whereas visual stimulations in the right visual field were eccentric. The authors first compared the event-related potentials (ERPs) in response to straight-ahead vs eccentric stimulations and found that, although retinal inputs were identical in the two conditions, peak amplitudes of the P1 bilateral (~ 140 ms after stimulus onset), N1 bilateral (~ 200 ms after stimulus onset) and P2–P3 (~ 300 ms after stimulus onset) components were significantly higher for straight-ahead stimuli (see their Fig. 2). Peak latencies did not differ between the two conditions. These effects are consistent with a gain modulation of the responses, in line with the previous findings in macaque electrophysiology (see Sect. "First findings in electrophysiology") and human fMRI (see above).

To determine the first occurence of these straight-ahead effects in their data, Bogdanova et al. (2020) subsequently computed the difference between EEG responses evoked by stimulations in the left and right visual hemi-fields. This operation permits to highlight another ERP component, the P1 contralateral, which peaks around 80 ms after stimulus onset (see Fig. 3B, upper panel) and which is believed to reflect responses from extrastriate visual areas V3, V3A and V4 (Di Russo et al. 2002). While the latency of the P1 contralateral component was independent of the condition, its amplitude was significantly more important for straight-ahead stimuli, as early as 65–70 ms after stimulus onset (see the global field powers in the upper-right panel of Fig. 3B.

To go further along this direction, the authors performed a second experiment which was specifically designed to test whether the straight-ahead direction also affects the earliest observable visual ERPs. It was based on the same experimental design except that the flickering checkerboard wedge was randomly presented at 10° of eccentricity within one of the four visual quadrants (Fig. 3B, lower-left panel). Bogdanova and colleagues subsequently computed the difference between EEG responses to stimulations in the upper vs lower visual field. This operation permits defining the C1 component which, respectively, arises and peaks around 55 and 75 ms after stimulus onset and which is believed to reflect the first feedforward visual processes in early visual areas (i.e., in V1, V2 and V3, see Ales et al. 2010). The peaks and latencies of this C1 component were not significantly different for straight-ahead vs eccentric stimulations (see Fig. 3B, lower-right panel). Stronger amplitudes for straight-ahead stimulations only appeared later, notably for the P1 bilateral, N1 bilateral and P2–P3 components, in agreement with the results of the first experiment. Finally, it is also worth reporting that in these two EEG experiments, power in the alpha band before stimulus onset was dependent on eye position: it was reduced in the hemisphere ipsilateral to gaze direction and thus contralateral to the straight-ahead direction. This finding is in line with previous EEG studies which showed that pre-stimulus alpha power suppression is associated with enhanced evoked activity (Van Dijk et al. 2008) and modifications of the P1 amplitude (Fellinger et al. 2011). It suggests that the privileged processing of the straight-ahead direction could be associated with a modification of the spontaneous activity that emerges as early as the eyes move toward an eccentric position.

Altogether, these two studies demonstrated that straight-ahead effects are also observable in the human occipital cortex (notably in area V1, Strappini et al. 2015) as early as 65–70 ms after stimulus onset (Bogdanova et al. 2020). If these results are generally compatible with those observed in monkey electrophysiology, (Durand et al. 2010, see Sect. "First findings in electrophysiology"), further studies will be necessary to determine whether the earlier effects observed in macaque (i.e., straight-ahead facilitations appear at about 40 ms after stimulus onset in this species) reflect a methodological (single/multi-unit recordings vs scalp EEG) and/or a species difference. The recent development of techniques such as intracranial recordings in implanted epileptic patients (De Jong et al. 2016) and/or monkey EEG (Sandhaeger et al. 2019) could permit to address this issue by making it possible to compare the effects obtained in the two species from the same methodological approach.

Studies in human (fMRI and EEG, see "Effects at the macroscopic level") and non-human (single and multi-unit recordings in macaque, see "First findings in electrophysiology") primates showed consistent increases in the neural responses to straight-ahead stimuli. What is the purpose of these specific gain modulations? In this section, we describe two studies that investigated behavioural consequences of this privileged visual processing of the straight-ahead direction.

In a first study, Durand et al. (2012) used a visual detection task to examine whether objects presented in the peripheral field of view elicit faster reaction times (RTs) when they are located straight-ahead rather than eccentric with respect to the head/body midline. Their stimulus was a vertically oriented Gabor patch displayed along the horizontal meridian. They used a 2 by 2 factorial design with retinotopic position (± 10°) and gaze direction (10° leftward or rightward) as main factors (see Fig. 4A). Participants (n = 21) were instructed to maintain their gaze on a central fixation cross and to detect and report as quickly as possible the brief apparitions of the Gabor patch with a button press. Importantly, the orientation of the screen was adapted for leftward and rightward fixation so that it remains perpendicular to the gaze axis (see their Fig. 2). This permitted to equalize the distances of all the targets from the observers. The analyses of the data revealed that during leftward gaze, median RTs were shorter for targets presented in the right visual field for 19 out of 21 participants. On the other hand, during rightward gaze, median RTs were shorter for targets presented in the left visual field for 18 out of 21 participants (see Fig. 4B). A statistical analysis based on a two-way RM ANOVA confirmed that neither retinotopic position nor gaze direction had a significant impact on RTs. The interaction term was nonetheless highly significant, revealing a clear effect of the target’s location with respect to the head/body axis. Across all the participants, median RTs were found to be about 10 ms shorter for straight-ahead vs for eccentric targets (Fig. 4C).

Importantly, gaze direction was recorded with an eye-tracker in all the participants and the analyses of these oculomotor data confirmed that the results were not driven by biases in ocular fixation (see Fig. 4 in Durand et al. 2012).

In their study, Durand et al. (2012) performed three important additional experiments. They first showed that their results remained unchanged under monocular viewing, as was also observed in monkey electrophysiology (see Durand et al. 2010). It demonstrates that the shorter RTs observed for straight-ahead stimuli are not caused by differences between the binocular (notably vertical) disparities contained in the two experimental conditions. In a second control experiment, they showed that straight-ahead facilitations were still observable when the subjects were involved in a dual task (to detect brief dimming of the fixation point). It suggests that the privileged processing of the straight-ahead direction does not require full attentional resources. We will further comment on the involvement of attention in straight-ahead facilitations in the discussion. Finally, in a third control experiment, participants were involved in the same dual task but the Gabor patches were presented along the upper vertical meridian (10° above the fixation point) and gaze direction was manipulated parametrically (− 10°, − 5°, 0°, 5° and 10°). Shorter RTs were always observed for Gabor patches presented straight-ahead (i.e., when ocular fixation was at 0°). It demonstrates that the straight-ahead preference is not caused by objects presented contralaterally to gaze direction. It also shows that this effect can be generalized to stimuli presented along the vertical meridian, in line with the neuroimaging (fMRI) finding described in Sect. "Effects at the macroscopic level" (Strappini et al. 2015).

In a following study, Durand and colleagues (Camors et al. 2016) addressed the potential impact of the privileged visual processing of the straight-ahead direction on oculomotor behaviour. Saccadic eye movements bringing the eyes toward the straight-ahead direction (centripetal saccades) are known to be initiated faster (Paré and Munoz 2001) and to reach higher peak velocity (Pelisson and Brablanc 1988) than saccades steering the eyes away from that direction (centrifugal saccades). However, it was generally admitted that the faster dynamics of centripetal saccades had motor origins, either linked to their privileged preparation and/or easiest execution. To investigate the sensory impact of presenting a saccadic target along or away from the straight-ahead direction, the authors used both pro- and anti-saccadic tasks: participants were instructed to move the eyes either toward or away from a suddenly appearing peripheral visual target. Such a design (Fig. 5A) allows dissociating the motor influence of the saccade’s direction (centripetal vs centrifugal) and the sensory influence of the visual target’s location (straight-ahead vs eccentric), since both centripetal and centrifugal saccades could be elicited by straight-ahead or eccentric targets. Centripetal pro-saccades were initiated earlier than centrifugal ones (see Fig. 5B, upper panel), as previously described by others (Laurutis and Robinson 1986). Importantly, results were reversed for anti-saccades (Fig. 5B, lower panel): centripetal anti-saccades were initiated later than centrifugal ones. What centripetal pro-saccades and centrifugal anti-saccades have in common is the fact that they were both triggered by straight-ahead targets, thus revealing that their faster initiation has a sensory rather than a motor origin. Interestingly, in both the pro- and anti-saccade tasks, centripetal saccades always reached higher peak velocity than centrifugal ones, ruling out a sensory origin for the faster execution. Overall, these results indicate that both sensory and motor factors contribute to the superior temporal properties of centripetal saccades: they are executed faster because of facilitated motor preparation/execution, but their faster initiation is explained by the privileged visual processing of visual elements located straight-ahead.

Interestingly, the execution of centripetal and centrifugal saccades has been shown to evoke differential neural activity in the superior colliculus of both monkeys (Paré and Munoz 2001) and humans (Krebs et al. 2010). Yet, it would be hazardous to make a link with the visual straight-ahead bias described here. In monkeys, modulations of collicular activity precede the appearance of the visual target and they only concern the fast saccades. In humans, reduced BOLD activations for centripetal saccades are observed in the colliculus, but not in the parietal and frontal eye fields. Although these results suggest that the observed modulations have an oculomotor rather than a visual origin, we have shown the necessity to introduce both pro- and anti-saccade tasks to disentangle the respective contributions of both components.

If the studies described in this review focused on area V1, it is likely that several other visual areas are also engaged in the privileged processing of the straight-ahead direction. Indeed, the human fMRI recordings (Strappini et al. 2015, see "Behavioural effects") showed strong straight-ahead effects in V2. The same study found significant interactions between visual stimulation and eye position in several retinotopic areas which suggests their possible implication in egocentric processing, in line with previous fMRI results (Deutschländer et al. 2005). In EEG, the straight-ahead direction is associated with a stronger amplitude of the contralateral P1 component (Bogdanova et al. 2020), which is believed to reflect neural responses from mid-level visual areas V3, V3A and V4 (Di Russo et al. 2002). It is, therefore, likely that the privileged processing of straight-ahead stimuli is ubiquitous in the primate visual system, which raises the question of how and where it emerges. Classically, it is hypothesized that gaze direction modulations are mediated by different extraretinal signals either proprioceptive, or efference copy (corollary discharge) or both. Indeed, early visual cortex (and notably area V1, where straight-ahead effects were first measured) receives proprioceptive projections whose implications for spatial cognition are well documented, notably by developmental studies (Steinbach 1987; Buisseret 1995; Donaldson 2000 for reviews). Its activity is also influenced by the efference copy, especially during eye movements, to account for visual stability (Morris and Krekelberg 2019; Wurtz 2008 for review). In addition, a visual signal, the vertical binocular disparities that exist at peripheral eccentricities, could also be used to recover the 3-D space characteristics with or without an extraretinal source of information on the position of the eyes (Mayhew and Longuet Higgins 1982; Garding et al. 1995). If vertical disparities are actually encoded in V1 (Durand et al. 2002, 2007), their involvement in straight-ahead modulations was discarded as these modulations persist under monocular viewing (Durand et al. 2010). The origin of the straight ahead modulations might be accounted for by the early integration of these extra-retinal signals (proprioception, efference copies) with visual inputs in primary visual cortex and even subcortical areas like the LGN (Donalson and Dixon 1980; Lal and Friedlander 1989), where a neural mechanism for localization of visual targets would exist, leading to a possible feedforward propagation of the straight-ahead preference. Alternatively, this preference might emerge in higher order associative areas, with a feedback propagation toward the early visual cortex that would raise when gaze direction is changed.

Whatever the feedforward or feedback nature of the propagation of the straight-ahead preference, this phenomenon can be considered as a mechanism of spatial attention: it serves allocating more neuronal resources for the visual analysis of a behaviourally relevant portion of the surrounding space. In line with this idea, the neural gain increases measured for straight-ahead stimuli in primary visual cortex (Strappini et al. 2015) and at early latencies (Bogdanova et al. 2020) are similar to those observed when manipulating attention (see, e.g., Kastner et al. 1999 or Liu et al. 2005 for fMRI studies and Hillyard and Anllo-Vento 1998 for EEG findings). Classically, spatial attention mechanisms fall into one of 2 classes. Endogenous attention refers to internal mechanisms in which attention is deployed voluntarily toward a portion of space selected on the basis of prior knowledge and cognitive operations (such as task demand, probability of relevance, etc.). As such, it is considered as a top-down (or feedback) mechanism. Exogenous attention, by contrast, is deployed reflexively toward locations in which unexpected and salient external events occur. Exogenous attention is generally seen as a bottom-up (feedforward) mechanism. Interestingly, the gain increases for straight-ahead stimuli described in the present review fail to completely fit within either of the 2 classes but rather, they share some characteristics with both of them. Because the signals associated with eye position (be it proprioceptive or efference copy, see above) are internal and trigger a privileged processing of a relevant portion of the surrounding space, straight-ahead modulations can be seen as endogenous. On the other hand, straight-ahead effects can be related to exogenous attention, as they are processed involuntarily and regardless of the ongoing task (see, e.g., Corbetta and Shulman 2002 for review). Indeed, in the fMRI study of Strappini and colleagues (2015), participants were either instructed to passively view the stimulus or to perform a task designed to strengthen covert visual attention on the stimulus (they had to detect the rare appearance of a letter within a continuous stream of letters superimposed on the rotating wedge). This manipulation did not affect the results as significant straight-ahead effects were observed in both cases. In our behavioural study (Durand et al. 2012), shorter reaction times for straight-ahead stimuli were maintained when participants performed a dual task (i.e., to detect brief dimming of the fixation point). These results demonstrate that straight-ahead effects remain significant even when attention is deviated toward another task. In the end, the straight-ahead effect could actually rely on a third class of attention mechanisms that share properties with both endogenous and exogenous attention, i.e., the experience dependent control of attention or history-driven selection as recently proposed by several studies (Theeuwes 2018, 2019; Chelazzi and Santandrea 2018). In that scheme, visual attention can be shaped by previous interactions of the individual with the environment through various forms of experience-dependent plasticity prioritizing the processing of specific spatial locations. This history-based selection has been shown to establish relatively quickly in various contexts including reward, statistical learning, priming and acquired fear. Fundamental learning processes influence what aspects of the environment stand out to guide attention and action in service of the ultimate goal of survival (Todd and Manaligod 2018 for review). Experience might be effective in inducing plasticity in the brain so as to shape the basic architecture of visual processing with multiple priority maps favoring objects and locations that are relevant for motor (including oculomotor) actions (Chelazzi and Santandrea 2018). Straight-ahead direction is a potential candidate for participating in this mechanism within different areas of the visual hierarchy, starting as early as the striate cortex going through the oculomotor-motor network in an egocentric frame of reference.

The experiments described in this review offer ample evidence that when the eyes are directed toward eccentric locations, peripheral vision provides a privileged processing for the visual objects we are facing. However, in all these studies, the head and trunk of the participants were always aligned, implying that the straight-ahead direction was defined similarly by both the head and trunk orientations. What happens when the gaze direction implies both eye and head movements, so that the head and trunk are not aligned anymore? Will peripheral vision allocate more resources to objects located in front of the head, in front of the trunk or in some intermediate directions? This issue actually ties in with the more general question of which part(s) of the egocentric space benefit from privileged visual processing by peripheral vision. The fMRI study of Strappini and colleagues (2015) indicates that along the vertical dimension, the straight-ahead direction seems to be defined by the eyes level, since maximum V1/V2 BOLD responses were observed for checkerboard wedges presented at eyes level (see Fig. 3A). However, the definition of the straight-ahead direction when the head and trunk are not aligned is ambiguous only along the horizontal dimension. Interestingly, in patients suffering from cervical dystonia, a neurological disorder possibly caused by a malfunctioning of the head neural integrator (Shaikh et al. 2016), neither the visual objects aligned with the head or those aligned with the trunk evoked faster reaction times (Amlang et al. 2017). Although this finding does not disentangle whether the impaired preference for straight-ahead stimuli is a cause or the consequence of cervical dystonia, it suggests that normal registration of the head and trunk axes is essential for this phenomenon to occur. To clarify the major anchor of the straight-ahead direction (head-centered and/or trunk-centered), future work should, therefore, also manipulate head orientation.

Our team also established that besides the direction of gaze, the viewing distance could also trigger gain modulations in a large majority of V1 neurons (Trotter et al. 1992, 1996). Such finding was confirmed and extended by other groups (see, e.g., Dobbins et al. 1998; Rosenbluth and Allman 2002). Rosenbluth and Allman (2002) notably combined changes in viewing distance and in gaze direction and confirmed that both parameters strongly impact the gain of most V1, V2 and V4 neurons. Interestingly, Dobbins and colleagues (1998) found that most V1/V2 and V4 neurons also showed a preference for short viewing distances, a finding further confirmed for V4 neurons (Rosenbluth and Allman 2002), but not for V1/V2 (Rosenbluth and Allman 2002; Trotter et al. 1992, 1996). One can note, however, that Trotter and colleagues reported higher rates of spontaneous activity for shorter distances (Trotter et al. 1992, 1996). It remains to be investigated whether neurons in charge of the peripheral visual field could actually exhibit a more pronounced preference for short distances corresponding to the peripersonal space. Such a finding, combined with the straight-ahead preference documented in this review, would suggest that the privileged portion of egocentric space forms a volume located straight-ahead within the peripersonal space. A better characterization of this volume (its properties) constitutes an important lead for future research on this topic.