We all have a sense that stress affects the way we see and evaluate our surroundings—but does it help or hurt us? In the literature on how stress affects perception, there are some conflicting reports. On the one hand, stress appears to improve our ability to discriminate visual information: better discrimination means better ability to detect a threat in the environment, so stress helps us out by putting the brain on high alert. This is useful for, say, walking through sketchy parts of town at night. But anyone who has ever frozen up during a big test can attest to a very different experience: the words themselves may be perfectly visible, but their meanings hardly take as you scan the page. So why the disconnect?One possibility is that stress has differential effects on visual discrimination and the deeper processes that allow us to act on what we see. This is a plausible theory, considering that perceiving visual information and goal-oriented processing rely on different systems in the brain. Processing of visual sensory information occurs quickly (within the first 100-200 msec, a.k.a.”early”), while directing attention is a slower process that is mediated by higher-level regions, such as prefrontal cortex (PFC). Note: by “slower”, I mean within the first 300-500 msec, a.k.a. “late”. This means that, hypothetically, a stress response could enhance early visual processing of the environment through one connection while dulling goal-directed processing via another route.
A recent study (Shackman & Maxwell et al., 2011) tested this theory by measuring how stress affected both early and late neural responses in participants who performed a visual discrimination task. In order to stress out the participants, the researchers delivered painful electric shocks at random times, but only during specific portions of the experiment (threat blocks); fortunately, the participants were spared for the other half of the experiment (safe blocks). To compare the timing of stress effects on perception versus goal-directed processing, the authors recorded electrical activity from the scalp using the event-related potential (ERP) technique. Importantly, this method allowed them to dissociate early visual versus late goal-directed responses with millisecond resolution. This comparison revealed that the amplitude of an early response associated with sensory processing (called the N1 component) was greater during threat versus safe blocks, meaning that stress enhanced visual perception. In contrast, the authors found the opposite effect when comparing the amplitude of a late, goal-oriented response (called the P3 component), which was reduced during threat versus safe blocks, indicating that goal-directed attention suffered under stress.
This finding could explain the “deer in headlights” look I often get from students when I put them on the spot:
a threatening situation (targeting a student) triggers an increased vigilance response (paralysis, wide-eyes, and enhanced visual discrimination) and a decreased orienting response (significant delay in answering question. Or none at all.). Neurally speaking, the authors suggest that when the brain experiences stress, visual processing, which is normally under the control of executive areas such as PFC, becomes hijacked by emotional regions such as the amygdala. While this hypersensitivity increases vigilance and detection, the visual system then has a heightened response to everything, including non-threatening information. For healthy people, this may not be so bad and could even be an advantage, (especially when driving); but consider patients dealing with post-traumatic stress disorder, for whom non-threatening stimuli can trigger more severe reactions.
The real downside to this shift in processing is that it comes at the cost of a decreased ability to control what we attend to. Evidence for this effect came not only from the dampened P3 response under threat, but also from an analysis of performance on the discrimination task. While participants responded with a similar speed during threat and safe blocks of the experiment overall, the authors did show an interesting relationship between P3 amplitude, stress and reaction time stemming from the right frontal area of the brain: the greater the threat-induced attenuation to an individual’s P3 response, the slower they were to respond to the stimuli. In other words, there was decreased task-orienting under stress, but only for those who experienced a strong enough decrease in their P3 response.
This last result confirms what we already know: some people can deal with stressful situations better than others. E.R. doctors are notorious for their ability to keep their cool under high-pressure situations, whereas some people succumb to stress to the point of an anxiety disorder. But what is it that makes us resilient to the amygdala take-over that stress can induce? This last piece of the puzzle is crucial if we want to find a way to preserve attentional focus in the face of stress, both for the everyday test-taker and for clinical populations.
Shackman AJ, Maxwell JS, McMenamin BW, Greischar LL, & Davidson RJ (2011). Stress Potentiates Early and Attenuates Late Stages of Visual Processing. The Journal of neuroscience : the official journal of the Society for Neuroscience, 31 (3), 1156-1161 PMID: 21248140
