Adaptation duration affects the spatial selectivity of facial aftereffects

AbstractAdaptation processes in human early visual cortical areas are sensitive to the exposure time of the adaptor stimulus. Here we investigated the effect of adaptation duration at the higher, shape-specific stages of visual processing using facial adaptation. It was found that long-term (5s) adaptation evokes facial aftereffects consisting of a position invariant as well as a position-specific component. As a result of adaptation to a female face, test faces were judged more masculine when they were displayed in the same location as the female adaptor face, as compared to that when they were presented in the opposite visual hemifield. However, aftereffects evoked by short-term (500ms) adaptation were found to be entirely position invariant. In accordance with these behavioral results, we found that the adaptation effects, measured on the amplitude of the N170 ERP component consisted of a position-specific component only after long-term, but not after short-term adaptation conditions. These results suggest that both short and long exposure to a face stimulus leads to adaptation of position invariant face-selective processes, whereas adaptation of position-specific neural mechanisms of face processing requires long-term adaptation. Our findings imply that manipulating adaptation duration provides an opportunity to specifically adapt different neural processes of shape-specific coding and to investigate their stimulus selectivity

Remote Effects of OFA Disruption on the Face Perception Network Revealed by Consecutive TMS-FMRI

The face perception system is comprised of a network of connected regions including the middle fusiform gyrus (“fusiform face area” or FFA), the inferior occipital gyrus (“occipital face area” or OFA), and the posterior part of the superior temporal sulcus. These regions are typically active bilaterally but may show right hemisphere dominance. The functional magnetic resonance imaging (fMRI) response of the right FFA is normally attenuated for face stimuli of the same compared to different identities, called fMR-adaptation. The recovery in fMRI signal, or release from fMR-adaptation, for faces of different identities indicates that the neural population comprising the FFA is involved in coding face identity. Patients with prosopagnosia who are unable to visually recognize faces and who show right OFA damage, nonetheless show face-selective activation in the right FFA (Rossion et al., 2003; Steeves et al., 2006). However, the sensitivity to face identity is abnormal in the right FFA and does not show the typical release from adaptation for different face identities (Steeves et al., 2009). This indicates that in these patients the FFA is not differentiating face identity and suggests that an intact right OFA is integral for face identity coding. We used offline repetitive transcranial magnetic stimulation (TMS) to temporarily disrupt processing in the right OFA in healthy subjects. We then immediately performed fMRI to measure changes in blood oxygenation level dependent (BOLD) signal across the face network using a face fMR-adaptation paradigm. We hypothesized that TMS to the right OFA would induce abnormal face identity coding in the right FFA, reflected by a decreased adaptation response. Indeed, activation for different but not same identity faces in the right FFA decreased after TMS was applied to the right OFA compared to sham TMS and TMS to a control site, the nearby object-selective right lateral occipital area (LO). Our findings indicate that TMS to the OFA selectively disrupts face but not butterfly identity coding in both the OFA and FFA. Congruent with mounting evidence from both patients and healthy subjects, here we causally demonstrate the importance of the often-overlooked OFA for normal face identity coding in the FFA

Birds of a Feather Flock Together: Experience-Driven Formation of Visual Object Categories in Human Ventral Temporal Cortex

The present functional magnetic resonance imaging study provides direct evidence on visual object-category formation in the human brain. Although brain imaging has demonstrated object-category specific representations in the occipitotemporal cortex, the crucial question of how the brain acquires this knowledge has remained unresolved. We designed a stimulus set consisting of six highly similar bird types that can hardly be distinguished without training. All bird types were morphed with one another to create different exemplars of each category. After visual training, fMRI showed that responses in the right fusiform gyrus were larger for bird types for which a discrete category-boundary was established as compared with not-trained bird types. Importantly, compared with not-trained bird types, right fusiform responses were smaller for visually similar birds to which subjects were exposed during training but for which no category-boundary was learned. These data provide evidence for experience-induced shaping of occipitotemporal responses that are involved in category learning in the human brain

Phase noise reveals early category-specific modulation of the event-related potentials

Previous studies have found that the amplitude of the early event-related potential (ERP) components evoked by faces, such as N170 and P2, changes systematically as a function of noise added to the stimuli. This change has been linked to an increased perceptual processing demand and to enhanced difficulty in perceptual decision making about faces. However, to date it has not yet been tested whether noise manipulation affects the neural correlates of decisions about face and non-face stimuli similarly. To this end, we measured the ERPs for faces and cars at three different phase noise levels. Subjects performed the same two-alternative age-discrimination task on stimuli chosen from young–old morphing continua that were created from faces as well as cars and were calibrated to lead to similar performances at each noise-level. Adding phase noise to the stimuli reduced performance and enhanced response latency for the two categories to the same extent. Parallel to that, phase noise reduced the amplitude and prolonged the latency of the face-specific N170 component. The amplitude of the P1 showed category-specific noise dependence: it was enhanced over the right hemisphere for cars and over the left hemisphere for faces as a result of adding phase noise to the stimuli, but remained stable across noise levels for cars over the left and for faces over the right hemisphere. Moreover, noise modulation altered the category-selectivity of the N170, while the P2 ERP component, typically associated with task decision difficulty, was larger for the more noisy stimuli regardless of stimulus category. Our results suggest that the category-specificity of noise-induced modulations of ERP responses starts at around 100 ms post-stimulus

Babies and Brains: Habituation in Infant Cognition and Functional Neuroimaging

Many prominent studies of infant cognition over the past two decades have relied on the fact that infants habituate to repeated stimuli – i.e. that their looking times tend to decline upon repeated stimulus presentations. This phenomenon had been exploited to reveal a great deal about the minds of preverbal infants. Many prominent studies of the neural bases of adult cognition over the past decade have relied on the fact that brain regions habituate to repeated stimuli – i.e. that the hemodynamic responses observed in fMRI tend to decline upon repeated stimulus presentations. This phenomenon has been exploited to reveal a great deal about the neural mechanisms of perception and cognition. Similarities in the mechanics of these two forms of habituation suggest that it may be useful to relate them to each other. Here we outline this analogy, explore its nuances, and highlight some ways in which the study of habituation in functional neuroimaging could yield novel insights into the nature of habituation in infant cognition – and vice versa

Senescent Changes in Orientation, Frequency, and 3-D Slant and Shape Perception

The ability to perceive the 3-Dimensional world is effortless despite the fact that the input to the visual system is 2-Dimensional. Attempts to derive biologically plausible models of shape from texture have focused on how changes in orientation and spatial frequency information are processed based on the response properties of primary visual cortex (V1) neurons. However, the relative contributions of orientation and spatial frequency information in detecting slant and shape from 3-D surfaces are not well understood. Additionally, in senescence, changes in optical components of the eye result in reduced frequency sensitivity, but whether concurrent neurophysiological changes affect the ability to discriminate orientation, and whether there is a resulting effect on form processing with age have remained unclear. An initial set of psychophysical experiments administered to younger adults showed that changes in orientation (or orientation modulations, OMs) dictated 3-D slant perception at shallow and steep slants, while changes in frequency (or frequency modulations, FMs) were only effective at steeper slants. This effect of OMs dictating slant and shape percept remained present even if a surface contained a texture with OM and FM components specifying inconsistent degrees of surface slant or curvature. Three additional psychophysical experiments were conducted to assess age-related changes in orientation and shape discrimination between younger and older observers. Consistent with previous findings in the literature, the older observers had significantly higher contrast thresholds than the younger group. Orientation discrimination thresholds were significantly higher for older observers when stimulus contrast was expressed as absolute values. However, when thresholds were evaluated in terms of multiples of detection threshold (to normalize stimuli for visibility across observers), age-related differences in orientation discrimination were not observed. Similarly, when observers performed a shape detection task, no significant difference was observed in shape detection thresholds across different spatial frequencies when age-related differences in contrast sensitivity were taken into account. However, when observers were given a shape discrimination task, older observers showed a significantly higher discrimination threshold at the highest spatial frequency even when thresholds were normalized for visibility. These findings suggest that while contrast detection thresholds increase with age, orientation and shape processing remain largely preserved. This suggests that in the context of degradations in optical and neural inputs, the overall percept of orientation and shape remains preserved with age, consistent with findings in several other areas in the visual system (e.g., color vision, perceived contrast). Our results suggest that the preservation of orientation and frequency perception with age at least partially contribute to the stability of 3-D shape perception, for stimuli in which orientation and frequency changes are cues for 3-D shape