Performing under pressure: stress and cognitive function

Abstract

The way in which cognitive functioning is affected by stressors is an important area of research for applied ethologists because stress caused by captive conditions may disrupt cognitive processes and lead to welfare and husbandry problems. Such problems may be minimised through an understanding of the links between stress and cognition. The effects of stress on cognitive function have been studied in disciplines ranging from human perceptual psychology to animal neuroscience. The aim of this paper is to provide an introduction to this research, focusing on the effects of stressors on attention, memory formation and memory recall. Findings from such a diverse literature with little apparent inter-disciplinary communication are inevitably complex and often contradictory. Nevertheless, some generalities do emerge. The idea that an inverted U-shaped relationship exists between an individual's state of stress or arousal and its ability to perform a cognitive task effectively, the so-called Yerkes–Dodson law, is commonly encountered. The law has limited explanatory value because it is unlikely that different stressors act on cognitive function via the same intervening, non-specific state. Furthermore, the law only provides a very general description of the relationship between stress and cognitive function. Empirical research on attention and memory processes reveals more specific findings. Stressors appear to cause shifts, lapses and narrowing of attention, and can also influence decision speed. These processes may be viewed as serving an adaptive role helping the animal to search for and scrutinise a source of danger. There is conflicting evidence as to whether hormones involved in the hypothalamic–pituitary–adrenal stress response play a part in these processes. These hormones and those involved in the sympathetic-adrenomedullary stress response do appear to play an important role in memory formation. Low or moderate concentrations of circulating glucocorticoids and catecholamines can enhance memory formation, while excessively high or prolonged elevations of these hormones can lead to memory disruption. The effects of stressors on memory recall are less clear. There is evidence for disruptive effects, and for facilitatory effects indicating state-dependent memory recall; events experienced under conditions of high arousal may be best recalled under similar conditions. Applied ethologists have the opportunity to extend work in this area, which often involves studies of single stressors/stress hormones acting in isolation and limited measures of cognitive function, by focusing on real-life husbandry stressors encountered by captive animals. This will yield fundamental information which also has direct relevance to animal welfare and management issues.

Introduction

The aim of this paper is to provide an introduction to research that investigates how stress affects cognitive function. This is a very broad question which raises the issues of how to define stress and cognition and whether stress-related states and cognitive processes can be treated as distinct, separate entities. For example, there is disagreement as to whether or not an organism's emotional responses to a stressor can occur independently of cognitive processes. Although these are important issues, they are beyond the scope of this paper and are discussed in detail elsewhere (e.g., Zajonc, 1980; Lazarus, 1982; Broom and Johnson, 1993; Nicol, 1995; Toates, 1995, Toates, 1997). Instead, I have chosen to take a pragmatic, empirical approach to this research area. In searching the literature, I have selected studies as being relevant to this topic if they investigate how apparently threatening and challenging stimuli (including manipulation of “stress hormones”) affect attention, learning and memory. Within these general boundaries, I have not defined the terms “stress” and “cognition” any more precisely but have used them simply as convenient labels which capture the essence of the area that I want to explore (cf. Rushen, 1986a).

Understanding how stressors modulate cognitive processes is important in its own right (e.g., Broadbent, 1971; Hockey et al., 1986), but I suggest that it may also be of value to applied ethology for the following reasons. If the stress of the housing and husbandry of farmed, laboratory and zoo animals impairs cognitive abilities such as learning and memory, this may lead to welfare and management problems. For example, the occurrence of aggression between previously familiar animals (e.g., Luescher et al., 1990; Mount and Seabrook, 1993) may be the result of stressful conditions, such as mixing or temporary removal from a group, disrupting individual recognition processes. Similarly, efficient husbandry and management will be hindered if animals fail to learn and remember the relative harmlessness of routine procedures such as weighing or veterinary inspection, and how to use equipment such as electronic feeders, or robotic milkers.

Choice testing is a widely used procedure in animal welfare research, and animal choices are usually interpreted from a motivational perspective as demonstrating the importance, value or aversiveness of particular resources or stimuli to the animal (e.g., Dawkins, 1990). However, it is also important to consider what might be termed the cognitive components of choice testing; for example, the learning and memory of an association between a location in a choice test apparatus and a resource or stimulus. If conditions of testing are stressful to the animal and impair learning and memory, this may disrupt choices and hence give spurious results with potentially misleading welfare implications. In a number of papers on pigs, sheep, cattle and deer, the suggestion has been made that agitated animals are indeed poorer at making clear-cut choices in preference or avoidance tests (Rushen, 1986b; van Rooijen and Metz, 1987; Pollard et al., 1994; Grandin et al., 1994; see Mendl et al., 1997).

Stress effects on cognitive function may also prevent us from gaining a true picture of the cognitive abilities of domestic animals; an important goal in applied ethology research (Dawkins, 1993; Nicol, 1996). For example, in experiments using detour behaviour tasks to investigate the ability of chicks to maintain memories of an object in the absence of sensory cues from that object (object permanence), Regolin et al. (1995)reported that this ability was masked when the chicks were tested under the stressful conditions of social isolation.

Understanding the relationship between stress and cognition may also allow the development of more effective methods for training animals, and for using behaviour therapy. For example, Walker et al. (1997)discuss how learning in the dog may be modulated by drugs which appear to alter anxiety or stress states.

Finally, from a welfare perspective, it is important to know whether stress experienced during early life, and long-term stress imposed by some husbandry conditions can lead to changes in the cognitive capacities of animals (e.g., learned helplessness, Overmier et al., 1980).