Impairments in social cognition-including recognition of facial expressions-are increasingly recognised as a core deficit in schizophrenia. It remains unclear whether other aspects of face processing ...(such as identity recognition) are also impaired, and whether such deficits can be attributed to more general cognitive difficulties. Moreover, while the majority of past studies have used picture-based tasks to assess face recognition, literature suggests that video-based tasks elicit different neural activations and have greater ecological validity. This study aimed to characterise face processing using video-based stimuli in psychiatric inpatients with and without psychosis. Symptom correlates of face processing impairments were also examined. Eighty-six psychiatric inpatients and twenty healthy controls completed a series of tasks using video-based stimuli. These included two emotion recognition tasks, two non-emotional facial identity recognition tasks, and a non-face control task. Symptoms were assessed using the Positive and Negative Syndrome Scale. Schizophrenia and bipolar disorder groups were significantly impaired on the emotion-processing tasks and the non-face task compared to healthy controls and patients without psychosis. Patients with other forms of psychosis performed intermediately. Groups did not differ in non-emotional face processing. Positive symptoms of psychosis correlated directly with both emotion-processing performance and non-face discrimination across patients. We found that identity processing performance was inversely associated with cognition-related symptoms only. Findings suggest that deficits in emotion-processing reflect symptom pathology independent of diagnosis. Emotion-processing deficits in schizophrenia may be better accounted for by task-relevant factors-such as attention-that are not specific to emotion processing.
Despite a long and productive history as a focus of research interest, the details of how humans detect motion in an image remain controversial. This debate has not been helped by the lack of a clear ...parametric description of motion discrimination for some of the more simple visual stimuli employed in the literature to date. With this in mind, in the present work, we examined a peculiarity observed in the perception of the motion of second-order (contrast-modulated) stimuli: Under certain stimulus conditions, there is a reversal in the perceived direction of motion of the pattern. The aim was to quantify this phenomenon, relate the reversal to forward (veridical) and ambiguous motion, and place the behavioral data in the context of the window of visibility model of spatiotemporal contrast sensitivity. The direction of motion of contrast-modulated patterns was measured as a function of temporal frequency and carrier contrast, under different critical stimulus conditions. The stimulus properties manipulated were spatial frequency, spatial-phase relationship of carrier and sidebands, color, duration, and, most critically, the retinal location of the stimulus. On a purely empirical basis, the data reconciled several conflicts in the recent literature. From a theoretical standpoint, the data were well explained by the window of visibility approach in the majority of conditions and were partially explained in the remaining conditions. The results raise some interesting questions about underlying motion detection mechanisms and the assumptions embodied in our approach to motion modeling and the visual system in general. Supplemental materials for this article may be downloaded from app.psychonomic-journals.org/content/supplemental.
The human visual system is much better at analysing the motion of luminance (black and white) patterns than it is at analysing the motion of colour patterns, especially if the pattern is presented ...very briefly or moves rapidly. We report here that observers reliably distinguish the direction of motion of a colour pattern presented for only 17 milliseconds, provided that the contrast is several times the threshold value (the contrast needed to detect the presence of the pattern). A control experiment, in which a static luminance 'mask' is added to the moving colour pattern, proves that discrimination of the direction of motion of these brief stimuli is colour-specific. The mask drastically impairs discrimination of the direction of motion of a luminance pattern, but it has little effect on a colour pattern. We conclude that the human visual system contains colour-specific motion-detection mechanisms that are capable of analysing very brief signals.
This paper measures the minimum velocity required to discriminate the direction of motion (the lower threshold of motion--LTM) for patterns which consisted of spatial variations in luminance, ...chromaticity or luminance contrast in an attempt to distinguish between the underlying directionally-selective mechanisms. The characteristics of these patterns can be defined as first-order/Fourier stimuli (luminance and chromatic gratings) or second-order/non-Fourier stimuli (contrast gratings or "beats"). Measurements for each pattern were made at durations ranging from 0.015 to 0.96 sec and at contrasts of 0.5 log units above detection threshold and 1.5 log units above the threshold for detecting the stationary pattern. Observers were able to discriminate the direction of motion in luminance gratings and high contrast chromatic gratings at all durations above 0.015 sec. The direction of motion of beats and low contrast chromatic gratings was indiscriminable until they had been presented for at least 0.12 sec. This was taken to indicate the existence of a fast-acting and a slow-acting system dealing with the first- and second-order patterns respectively. When defined on this basis, the chromatic stimulus acts as a first-order (luminance) pattern at high contrasts and a second-order (beat) pattern at low contrasts.
The data presented in this paper examine the ability of observers to detect a modulation in the contrast of chromatic and luminance gratings as a function of the carrier contrast, duration, and ...spatial frequency. The nature of the signal underlying this ability is investigated by examining both the paradigm used to make the measurement and the effect of grating masks on performance in the tasks. The results show that observers' ability to discriminate amplitude modulation from an unmodulated carrier is dependent on carrier contrast but only up to approximately 5-8 times carrier-detection threshold. Discrimination is, however, independent of spatial frequency 10-1 cycles per degree (cpd) component-frequency range, carrier color, and, most surprisingly, stimulus duration (1000-30 ms). This set of experiments compliments data from previous papers and assimilates many of the conclusions drawn from this previous data. There is absolutely no evidence for the existence of a distortion product mediating performance under any of the current conditions, and the data seriously question whether the visual system might use such a signal even if it does exist under more extreme conditions than those used here. The evidence suggests that the visual system detects variations in both chromatic and luminance contrast by means of a mechanism operating locally upon the spatial structure of the carrier.
Motion of contrast envelopes: peace and noise Cropper, S J; Johnston, A
Journal of the Optical Society of America. A, Optics, image science, and vision,
09/2001, Letnik:
18, Številka:
9
Journal Article
Recenzirano
We examined the effect of changing the composition of the carrier on the perception of motion in a drifting contrast envelope. Human observers were required to discriminate the direction of motion of ...contrast modulations of an underlying carrier as a function of temporal frequency and scaled (carrier) contrast. The carriers were modulations of both color and luminance, defined within a cardinal color space. Random-noise carriers had either binary luminance profiles or flat (gray-scale-white) or 1/f (pink) spectral power functions. Independent variables investigated were the envelope spatial frequency and temporal-drift frequency and the fundamental spatial frequency, color, and temporal-update frequency of the carrier. The results show that observers were able to discriminate correctly the direction of envelope motion for binary-noise carriers at both high (16 Hz) and low (2 Hz) temporal-drift frequencies. Changing the carrier format from binary noise to a flat (gray-scale) or 1/f amplitude profile reduced discrimination performance slightly but only in the high-temporal-frequency condition. Manipulation of the fundamental frequency of the carrier elicited no change in performance at the low temporal frequencies but produced ambiguous or reversed motion at the higher temporal frequencies as soon as the fundamental frequency was higher than the envelope modulation frequency. We found that envelope motion detection was sensitive to the structure of the carrier.
Cultures around the world organise stars into constellations, or asterisms, and these groupings are often considered to be arbitrary and culture-specific. Yet there are striking similarities in ...asterisms across cultures, and groupings such as Orion, the Big Dipper, the Pleiades and the Southern Cross are widely recognized across many different cultures. Psychologists have informally suggested that these shared patterns are explained by Gestalt laws of grouping, but there have been no systematic attempts to catalog asterisms that recur across cultures or to explain the perceptual basis of these groupings. Here we compile data from 27 cultures around the world and show that a simple computational model of perceptual grouping accounts for many of the recurring cross-cultural asterisms. Our results suggest that basic perceptual principles account for more of the structure of asterisms across cultures than previously acknowledged and highlight ways in which specific cultures depart from this shared baseline.
It has become apparent from recent work that the spatial frequency and orientation content of the first-order (luminance) carrier is very important in determining the properties of a second-order ...(contrast) modulation of that carrier. In light of this we examined whether there was any evidence for a motion aftereffect in one-dimensional second-order patterns containing only two sinusoidal luminance components: a spatial beat. The stimuli were either 1 cpd luminance sinusoids or 1 cpd luminance beats modulating a carrier sinusoid of 5 cpd. The magnitude of any motion aftereffect, or any directionally specific effect of adaptation, was measured for all combinations of first and second-order test and adapting patterns. Both flickering and non-flickering stimuli were used. The results indicate that a motion aftereffect is
only induced by first-order adapting stimuli, and likewise, is only measurable in first-order test stimuli. We find no evidence for any directionally specific effect of adaptation in second-order stimuli, whether the test is counterphased or otherwise. These results apparently conflict with recent reports of a second-order induced motion aftereffect, but are consistent with many other findings which show differences between the detection of motion for first and second-order stimuli. We conclude that the induction of a motion aftereffect for second-order stimuli is not a general result and is critically dependent upon (amongst other things) the local properties of the stimulus, including the spatial frequency and orientation content of the first-order carrier.
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