Although body size and shape misperception (BSSM) is a common feature of anorexia nervosa, bulimia nervosa and muscle dysmorphia, little is known about its underlying neural mechanisms. Recently, a new approach has emerged, based on the long-established non-invasive technique of perceptual adaptation, which allows for inferences about the structure of the neural apparatus responsible for alterations in visual appearance. Here, we describe several recent experimental examples of BSSM, wherein exposure to “extreme” body stimuli causes visual aftereffects of biased perception. The implications of these studies for our understanding of the neural and cognitive representation of human bodies, along with their implications for clinical practice are discussed.

Many individuals experience body-size and -shape misperception (BSSM). Body-size overestimation is associated with body dissatisfaction, anxiety, depression, and the development of eating disorders in individuals who desire to be thinner. Similar symptoms have been noted for those who underestimate their muscularity. Conversely, individuals with high body mass indices (BMI) who underestimate their adiposity may not recognize the risks of or seek help for obesity-related medical issues. Although social scientists have examined whether media representations of idealized bodies contribute to the overestimation of fat or underestimation of muscle, other scientists suggest that increases in the prevalence of obesity could explain body-fat underestimation as a form of renormalization. However, these disparate approaches have not advanced our understanding of the perceptual underpinnings of BSSM. Recently, a new unifying account of BSSM has emerged that is based on the long-established phenomenon of visual adaptation, employing psychophysical measurements of perceived size and shape following exposure to “extreme” body stimuli. By inducing BSSM in the laboratory as an aftereffect, this technique is rapidly advancing our understanding of the underlying mental representation of human bodies. This nascent approach provides insight into real-world BSSM and may inform the development of therapeutic and public-health interventions designed to address such perceptual errors.

Contrast masking from parallel grating surrounds (doughnuts) and superimposed orthogonal masks have different characteristics. However, it is not known whether the saturation of the underlying suppression that has been found for parallel doughnut masks depends on (i) relative mask and target orientation, (ii) stimulus eccentricity or (iii) surround suppression. We measured contrast-masking functions for target patches of grating in the fovea and in the periphery for cross-oriented superimposed and doughnut masks and parallel doughnut masks. When suppression was evident, the factor that determined whether it accelerated or saturated was whether the mask stimulus was crossed or parallel. There are at least two interpretations of the asymptotic behaviour of the parallel surround mask. (1) Suppression arises from pathways that saturate with (mask) contrast. (2) The target is processed by a mechanism that is subject to surround suppression at low target contrasts, but a less sensitive mechanism that is immune from surround suppression 'breaks through' at higher target contrasts. If the mask can be made less potent, then masking functions should shift downwards, and sideways for the two accounts, respectively. We manipulated the potency of the mask by varying the size of the hole in a parallel doughnut mask. The results provided strong evidence for the first account but not the second. On the view that response compression becomes more severe progressing up the visual pathway, our results suggest that superimposed cross-orientation suppression precedes orientation tuned surround suppression. These results also reveal a previously unrecognized similarity between surround suppression and crowding (Pelli, Palomares, & Majaj, 2004).

Two-stroke apparent motion offers a challenge to current theoretical models of motion processing and is thus a useful tool for investigating motion sensor input. The stimulus involves repeated presentation of two pattern frames containing a spatial displacement, with a blank inter-stimulus interval (ISI) at one of the two-frame transitions. The resulting impression of continuous motion was measured here using both direction discrimination and motion after-effect duration in order to assess the extent to which data using the two measures can be explained by a computational model without reference to attentive tracking mechanisms. The motion-energy model was found to offer a very good account of the psychophysical data using similar parameters for both tasks. The experiment was run under both photopic and scotopic retinal illumination. Data revealed that the optimum ISI for perceiving two-stroke apparent motion shifts to longer ISIs under scotopic conditions, providing evidence for a biphasic impulse response at low luminance. Best-fitting model parameters indicate that motion sensors receive inputs from temporal filters whose central temporal frequency shifts from 2.5 to 3.0 Hz at high retinal illuminance to 1.0–1.5 Hz at low retinal illuminance.

No abstract