Long-term recalibration of orientation perception
Pascal Mamassian and Adrien Chopin
In the tilt after-effect, adaptation to left-oriented bars leads to an illusory right tilt of a perfectly vertical bar. This result is often taken as evidence that the visual system recalibrates its expectations for vertical with stimuli presented during the adaptation period. We wondered whether expectations take their origin in some kind of ideal norm or instead are inferred from the environment. Observers were presented with series of Gabors that could have one of two orientations A and B separated by 40 degrees. A test Gabor was subsequently presented and observers had to judge whether its orientation was closer to A or B. Unbeknownst to the observer, the orientation of the test was always exactly in-between A and B. We found that the perceived orientation of the test Gabor was biased away from the orientations of recently presented stimuli (tilt after-effect) and more surprisingly, toward the orientations of stimuli presented further in the past. We propose that the remote past is used to estimate the world’s statistics and that this estimate becomes the reference. More specifically, orientation perception is recalibrated relative to remote percepts (up to 10 min in the past) rather than relative to an ideal norm.
Testing the Channel-Based Model of Duration Perception
William Curran, Christopher P Benton, Paul B Hibbard and Julie M Harris
The channel-based model of duration perception postulates the existence of neural mechanisms that respond selectively to a narrow range of stimulus durations centred on their preferred duration (Heron et al, Proc Royal Soc B, 279, 690-698). In principle the channel-based model could explain recent reports of adaptation-induced, visual duration compression effects (Johnston et al, Current Biology, 16, 472-479; Curran & Benton, Cognition, 122, 252-257); from this perspective duration compression is a consequence of the adapting stimuli being presented for a longer duration than the test stimuli. In the current experiment observers adapted to a sequence of moving random dot patterns at the same retinal position, each 340ms in duration and separated by a variable (500 – 1000ms) interval. Following adaptation observers judged the duration of a 600ms test stimulus at the same location. The test stimulus moved in the same, or opposite, direction as the adaptor. Contrary to the channel-based model’s prediction, test stimulus duration appeared compressed, rather than expanded, when it moved in the same direction as the adaptor. That test stimulus duration was not distorted when moving in the opposite direction further suggests that visual timing mechanisms are influenced by additional neural processing associated with the stimulus being timed.
Orientation tuning and normalisation in the tilt aftereffect
Peter Thompson and Elena Gross
The tilt aftereffect (TAE) is usually demonstrated by adapting to a grating tilted approximately 10 degrees from vertical, after which a vertical grating appears tilted in the opposite direction. During adaptation the perceived orientation of the adaptor shifts towards the ‘null’ vertical position. This has been called normalisation. The spatial frequency aftereffect (SFAE), often attributed to the same neural processes as the TAE, behaves in a very different fashion, with no null point and hence no normalisation. We have investigated the effects of tilt adaptation to 0 deg (vertical), 15 deg and 45 deg gratings on a range of test gratings from -90 to 90 deg in 15 deg steps, using an orientation matching procedure. Our results show that adaptation to 45 deg (and to a lesser extent 0 deg) produces aftereffects similar to those of the SFAE, with shifts in perceived tilt away from the adaptation orientation, and no normalisation – the signature of adaptation of multiple narrowly tuned channels. Adaptation to 15 deg produces an effect that suggests both strong normalisation towards vertical and evidence of multiple orientation channels. These results suggest that normalisation and channel adaptation may be thought of as separate and distinct processes.
Temporal frequency tuning of the cortical face-sensitive network for individual face perception
Francesco Gentile and Bruno Rossion
In the present study we investigated the temporal frequency tuning of face-sensitive areas with functional magnetic resonance imaging (fMRI). Six observers were tested in an adaptation paradigm where blocks of same or different faces were presented at 11 different frequencies (1, 2, 3, 4, 5, 6, 6.66, 7.5, 8.57, 10, 12 Hz). The right fusiform face area and right occipital face area showed an adaptation effect - difference between same and different faces –consistently peaking, on average, at 6Hz. This effect was due both to the quick drop of the signal during the transition from 4 to 6Hz in the same condition and to a larger response for different faces (different condition) which decreased for frequencies higher than 6Hz. Therefore individual face discrimination seems to be optimal when a face is presented at a rate of 6Hz. Interestingly, at this rate, the complete cycle of the sinusoidal stimulation of a face lasts about 170 ms which is also the latency of the earliest face identity adaptation effect as found on the face-sensitive N170 ERP component (Jacques et al., J Vis, 2007). Our results support the view that the human brain requires about 160 ms to process individual faces efficiently.
Global shape processing involves feature selective and feature agnostic coding mechanisms
Jason Bell, Mimosa Forsyth, David Badcock and Frederick Kingdom
Recent research and modelling proposes that a shape is accurately described by both the degree of curvature and the angular location (relative to shape-centre) of its curved parts; a shape orientation selective representation. We tested this proposition. Radial Frequency (RF) patterns were employed as stimuli since these can represent a range of familiar shapes, and are processed globally. We measured an RF amplitude after-effect (AE) as a function of the shape orientation difference between adapt and test patterns, of the same RF. For RF3 and RF4, AEs were largest when adapt and test patterns were the same orientation, and then linearly decreased as the adaptor was rotated away. Interestingly, AEs did not reduce to zero, instead plateauing significantly above zero. By contrast, when adapt and test were opposite luminance-polarity, AEs were reduced and orientation invariant. When opposite luminance-polarity AEs were subtracted from same luminance-polarity AEs, a difference emerged only for similar adapt and tests orientations; thus ruling out local orientation adaptation as an explanation for this sub-set of the AEs. Our findings provide evidence for two global shape mechanisms: one that is selective for shape orientation and luminance polarity, and one that is agnostic to these characteristics.
Stimulus probability and stimulus expectation affect the activity of face and house selective areas of the human brain independently
Gyula Kovács, Mareike Grotheer and Mark W. Greenlee
It has been shown that the probability of face repetitions influences the magnitude of repetition-related response reductions (Summerfield et al., 2008, Larsson and Smith, 2011), implying that perceptual expectations affect adaptation and repetition suppression processes in the human brain. An unresolved question, however, is whether the observed effects are due to the fulfilled perceptual expectations or to the altered stimulus probabilities that were used in the previous paradigms to elicit predictions. To address this question we varied the probability of prior face and house stimuli independently of the expectation of the subjects. Subjects’ perceptual expectations regarding the occurrence of stimulus category (faces/house) were manipulated by a cue (Egner et al, 2010). Subsequently, category probability was modulated during an adaptation phase (8 images, ET=250 ms/image, 80% faces/ 20% houses or the other way around) independently from the predictor cue. Finally subjects made a category decision on ambiguous overlapping face/house composites (Cziraki et al, 2010). We found significant target related fMRI adaptation (fMRIa) in the fusiform and occipital face areas when the adaptor had high probability when compared to low probability of faces. More importantly, although expecting faces increased the target related responses, it had no effect on the observed fMRIa. Our results suggest that it is rather stimulus probability than expectation that affects the repetition-related response reduction of face specific areas of the human brain.
Motor-visual effects in the recognition of dynamic facial expressions
Cristobal Curio, Martin Giese, Heinrich H. Bülthoff and Stephan de La Rosa
Current theories on action understanding suggest a cross-talk between the motor and the visual system during the recognition of other persons’ actions. We examined the effect of the motor execution on the visual recognition of dynamic emotional facial expressions using an adaptation paradigm. Previous research on facial expression adaptation has shown that the prolonged visual exposure to a static facial expression biases the percept of an ambiguous static facial expression away from the adapted facial expression. We used a dynamic 3D computational face model (Curio et al., 2010, MIT Press, 47-65) to examine motor-visual interactions in the recognition of happy and fearful facial expressions. During the adaptation phase participants 1) looked for a prolonged amount of time at a facial expression (visual adaptation); 2) executed repeatedly a facial expression (motor adaptation); 3) imagined the emotion corresponding to a facial expression (imagine adaptor). In the test phase participants always had to judge an ambiguous facial expression as either happy or fearful. We found an adaptation effect in the visual adaptation condition, and the reversed effect (priming effect) in the motor and imagine condition. Inconsistent with simple forms of motor resonance, this shows antagonistic influences of visual and motor adaptation.
A cross-modal, cross-dimensional mechanism for numerosity perception
Roberto Arrighi, Irene Togoli and David Burr
Previous research has demonstrated that numerosity perception is a primary attribute of the visual system susceptible, like most visual attributes, to adaptation (Burr and Ross, 2008, Curr Biol, 18(6), 425-8). Here we investigate adaptation to sequentially presented items. The results show that adapting to series of flashes presented at a frequency of 2, 4 or 8 Hz for about 40 secs, robustly affects the perceived numerosity of visual pulses (range 2-20) subsequently presented in a temporal window of 2 seconds. Adapting to low frequency (2Hz) increased the amount of subsequent flashes of about 15-20%, and to high frequency (8 Hz) decreased apparent numerosity; 4 Hz had no effect. Similar effects were observed for auditory stimuli. Most interestingly, adaptation also occurred cross-modally: adapting to auditory bursts affected visual numerosity, and vice versa. These results support the idea that similar mechanisms underpin numerosity perception in vision and audition, in both space and time.