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T Troscianko

Publications and source records attributed to T Troscianko.

18 recordsLinked to original sources
PubMed2000

The human visual system is optimised for processing the spatial information in natural visual images.

A fundamental tenet of visual science is that the detailed properties of visual systems are not capricious accidents, but are closely matched by evolution and neonatal experience to the environments and lifestyles in which those visual systems must work. This has been shown most convincingly for fish and insects. For mammalian vision, however, this tenet is based more upon theoretical arguments than upon direct observations. Here, we describe experiments that require human observers to discriminate between pictures of slightly different faces or objects. These are produced by a morphing technique that allows small, quantifiable changes to be made in the stimulus images. The independent variable is designed to give increasing deviation from natural visual scenes, and is a measure of the Fourier composition of the image (its second-order statistics). Performance in these tests was best when the pictures had natural second-order spatial statistics, and degraded when the images were made less natural. Furthermore, performance can be explained with a simple model of contrast coding, based upon the properties of simple cells in the mammalian visual cortex. The findings thus provide direct empirical support for the notion that human spatial vision is optimised to the second-order statistics of the optical environment.

Adult↗
PubMed2000

Influence of sensitivity on response bias in taste and audition.

A detection theoretic analysis was employed to examine sensitivity and response bias in two modalities. In Experiment 1, 6 tasters made same-different judgments about the concentration of either sucrose or quinine in pairs of tonic water samples. The beverages were colored, but color was not predictive of the concentration of the sweet or bitter ingredient. When same-different ratings were collapsed to approximate the outcome of a categorical decision, tasters with poorer sensitivity appear to have adopted more extreme response criteria than did tasters with greater sensitivity, irrespective of taste quality, color, or whether pairs of solutions comprised the same or different colors. In Experiment 2, 3 individuals discriminated pairs of 1000-Hz sinusoids differing in amplitude. Six amplitude differences were tested. Rating-scale versions of two paradigms: The single-interval yes-no task and the two-interval same-different task were used to measure sensitivity and bias. There was a preponderance of "same" responses in the same-different task. Estimates of bias obtained from collapsed ratings in both tasks were unaffected by sensitivity, but a consideration of the range over which sets of criteria were spread suggested a general tendency toward more conservative response biases as sensitivity declined.

Adult↗
PubMed1999

Mismatch negativity in the visual modality.

In the auditory system, the automatic detection of stimulus change provides a mechanism for switching attention to biologically significant events. It gives rise to the mismatch negativity (MMN) event related potential. It is unclear whether a similar mechanism exists in vision. To investigate this issue, evoked potentials were recorded to target stimuli in the centre of the visual field, and to frequent standard and infrequent deviant stimuli presented outside the focus of attention, in the peripheral field. Deviants evoked a more negative potential than standards 250-400 ms after the stimulus. The negativity, distributed over supplementary visual areas of occipital and posterior temporal cortex, was associated with the rarity of the deviants and not the physical features which distinguished them from standards. This negativity shares a number of characteristics with auditory MMN.

Adult↗
PubMed1998

Complete sparing of high-contrast color input to motion perception in cortical color blindness.

It is widely held that color and motion are processed by separate parallel pathways in the visual system, but this view is difficult to reconcile with the fact that motion can be detected in equiluminant stimuli that are defined by color alone. To examine the relationship between color and motion, we tested three patients who had lost their color vision following cortical damage (central achromatopsia). Despite their profound loss in the subjective experience of color and their inability to detect the motion of faint colors, all three subjects showed surprisingly strong responses to high-contrast, moving color stimuli--equal in all respects to the performance of subjects with normal color vision. The pathway from opponent-color detectors in the retina to the motion analysis areas must therefore be independent of the damaged color centers in the occipitotemporal area. It is probably also independent of the motion analysis area MT/V5, because the contribution of color to motion detection in these patients is much stronger than the color response of monkey area MT.

Adult↗
PubMed1998

Color and luminance information in natural scenes.

The spatial filtering applied by the human visual system appears to be low pass for chromatic stimuli and band pass for luminance stimuli. Here we explore whether this observed difference in contrast sensitivity reflects a real difference in the components of chrominance and luminance in natural scenes. For this purpose a digital set of 29 hyperspectral images of natural scenes was acquired and its spatial frequency content analyzed in terms of chrominance and luminance defined according to existing models of the human cone responses and visual signal processing. The statistical 1/f amplitude spatial-frequency distribution is confirmed for a variety of chromatic conditions across the visible spectrum. Our analysis suggests that natural scenes are relatively rich in high-spatial-frequency chrominance information that does not appear to be transmitted by the human visual system. This result is unlikely to have arisen from errors in the original measurements. Several reasons may combine to explain a failure to transmit high-spatial-frequency chrominance: (a) its minor importance for primate visual tasks, (b) its removal by filtering applied to compensate for chromatic aberration of the eye's optics, and (c) a biological bottleneck blocking its transmission. In addition, we graphically compare the ratios of luminance to chrominance measured by our hyperspectral camera and those measured psychophysically over an equivalent spatial-frequency range.

Color Perception↗
PubMed1997

Automatic segmentation and classification of outdoor images using neural networks.

The paper describes how neural networks may be used to segment and label objects in images. A self-organising feature map is used for the segmentation phase, and we quantify the quality of the segmentations produced as well as the contribution made by colour and texture features. A multi-layer perception is trained to label the regions produced by the segmentation process. It is shown that 91.1% of the image area is correctly classified into one of eleven categories which include cars, houses, fences, roads, vegetation and sky.

Automation↗
PubMed1996

Human colour discrimination based on a non-parvocellular pathway.

BACKGROUND: Traditionally, colour information is assumed to be carried by neural channels in the parvocellular pathway and to be encoded in an opponent manner, while other, non-parvocellular, spectrally non-opponent channels are thought to play no part in colour vision. But is the parvocellular pathway the only way that colours can be discriminated in human vision? We studied two patients with cerebral achromatopsia, who lack conscious colour perception but are nevertheless able to make use of colour information. In particular, we investigated whether, in these patients, colour discrimination is mediated by the parvocellular pathway. RESULTS: The achromatopsic patients carried out a forced-choice colour- and luminance-discrimination task, and showed clear evidence of unconscious colour processing, consistent with previous studies. We added different types of luminance noise to see when this unconscious colour information could be masked. The results of the colour-discrimination-with-noise and the brightness-non-additivity experiments showed a double-dissociation between patients. This indicates that, in one patient, unconscious colour discrimination may be subserved by a spectrally non-opponent mechanism, which does not have the characteristics of the parvocellular pathway and which is responsive to fast flicker. Spectral sensitivity, contrast sensitivity and motion perception experiments confirmed that this patient lacks a working opponent parvocellular system. The second achromatopsic patient showed evidence of a residual parvocellular system. CONCLUSIONS: Our results show that chromatic discrimination need not be mediated by neural mechanisms, the parvocellular system in particular, normally assumed to subserve conscious colour perception. Such discrimination may be mediated by a neural subsystem which responds to fast flicker, is spectrally non-opponent, and supports normal motion perception.

Aged↗
PubMed1996

Perception of emotion from dynamic point-light displays represented in dance.

It is well known that biological motion, as produced by point-light displays on a human body, gives a good representation of the represented body-eg its gender and the nature of the task which it is engaged in. The question is whether it is possible to judge the emotional state of a human body from motion information alone. An ability to make this kind of judgment may imply that people are able to perceive emotion from patterns of movement without having to compute the detailed shape first. Subjects were shown brief video clips of two trained dancers (one male, one female). The dancers were aiming to convey the following emotions: fear, anger, grief, joy, surprise, and disgust. The video clips portrayed fully lit scenes and point-light scenes, with thirteen small points of light attached to the body of each dancer. Half the stimuli were presented the right way up, while half were inverted. The subjects' task was to judge which emotion was being portrayed. Full-body clips gave good recognition of emotionality (88% correct), but the results for upright biological-motion displays were also significantly above chance (63% correct). Inversion of the display reduced biological-motion (but not full-body) performance to close to chance but still significantly above chance. A space-time analysis of the motion of the points of light was carried out, and was related to the discriminability of the different emotions. Biological-motion displays, which convey no information while static, are able to give a rich description of the subject matter, including the ability to judge emotional state. This ability is disrupted when the image is inverted.

Analysis of Variance↗
PubMed1992

A colour-flicker analysis of visual function in patients with retinal detachment.

Patients with a history of retinal detachment were examined by the following tests: (1) setting of an isoluminant match between yellow and green constituent squares of a checkerboard and (2) measurement of the flicker fusion frequency for a contrast-reversal of this yellow-green pattern. The results suggest that the test may provide useful data for assessing the severity of any damage to a given part of the retina. The fusion frequencies range from over 10 Hz (normal) to about 2 Hz for a severely impaired retina. The data for the relative amounts of red and green light needed to achieve a match rarely show any abnormality; thus, a standard anomaloscope test or pseudoisochromatic plates would not detect these deficiencies.

Adult↗
PubMed1991

The role of colour as a monocular depth cue.

Does colour information play a role in the perception of depth? Its input to stereopsis is weak, and it has been suggested that depth from monocular cues, such as texture gradients, is also abolished at isoluminance (colour contrast with no luminance contrast). We first investigated whether depth from texture gradients disappears at isoluminance. The percept remained unaltered. Further experiments revealed that certain colour gradients (at isoluminance) markedly affected the perceived depth. A gradient in saturation (e.g. red-to-grey) was particularly effective, whereas a red-green hue gradient had no effect on perceived slant. We concluded that colour information can be used by the visual system to encode depth, especially in situations where the visual environments is rich in cues which could be used to signal depth in this way.

Color Perception↗
PubMed1988

Phase discrimination in chromatic compound gratings.

Phase discrimination thresholds were measured for yellow/green isoluminant and non-isoluminant compound gratings in which the amplitude of the two components (f and 3f) was twice the detection threshold. The phase discrimination threshold at isoluminance was worse than in all the other conditions, which gave broadly similar threshold data. It is suggested that this is due to a positional uncertainty in the neural representation of the isoluminant stimuli.

Contrast Sensitivity↗
PubMed1988

Why do isoluminant stimuli appear slower?

There is ample evidence that the perception of movement, both real and apparent, is substantially impaired at isoluminance. Models of movement perception require spatial and temporal information about the stimulus. We ask whether changes at isoluminance result from a spatial or a temporal error or uncertainty. Reaction times to three kinds of stimulus were measured: (a) temporal stimuli, such as the onset of a square in a known location; (b) spatial stimuli, a vernier displacement of two squares; and (c) spatiotemporal stimuli, moving squares either starting or stopping. The results suggest that there is relatively little effect of isoluminance on purely temporal tasks (a). Longer reaction times were, however, obtained for detecting vernier offset (b). The reaction times to moving stimuli (c) were also slower at isoluminance to an extent that implies that perceived velocity at isoluminance is approximately 30% less than that seen at 8% contrast. The slowing of reaction times at isoluminance could be mimicked by adding random positional jitter to a nonisoluminant moving stimulus and also by presenting a low-contrast monochromatic stimulus. A simple explanation of the data is given in terms of a motion-detecting unit coupled to a temporal integrator. It is shown how such a unit can encode perceived velocity. The results of these experiments suggest that the neural coding of isoluminant stimuli is similar to that of low-contrast luminance stimuli and therefore that isoluminance may not be an effective method to find out whether specific visual mechanisms are color-blind.

Adult↗
PubMed1987

Perception of random-dot symmetry and apparent movement at and near isoluminance.

There have been conflicting reports on whether apparent movement in random-dot kinematograms is abolished at isoluminance. The present results suggest that it is, provided that dynamic (uncorrelated) surrounds are used, and the subject has to report the shape of the target rather than the presence of movement in an isolated portion of the target. On the other hand, perception of random-dot symmetry is still possible at isoluminance. The reason for this difference appears to be the need for exact-position information in movement but not symmetry perception. Control experiments suggest that the effects are not due to artefacts such as chromatic aberration in the eye.

Adult↗
PubMed1984

An assessment of two amplitude-compression hearing aid systems, especially in high ambient noise.

A critical test of the suitability of amplitude compression in hearing aid design must look at the performance of such devices with high levels of background noise. A series of tests was carried out, using both normal and hearing-impaired subjects and word-list test material. The results favour some forms of amplitude compression, even under these critical conditions. However, nonlinear compression ('Carrier Clipping') can reduce the signal-to-noise ratio to unity under adverse conditions. Under these circumstances, such compression impairs performance, though in most situations, it is beneficial.

Amplifiers, Electronic↗
PubMed1982

A given visual field location has a wide range of perceptive field sizes.

Increment thresholds were measured at the intersections of a modified Hermann Grid at several retinal locations and at photopic, mesopic and scotopic adaptation levels. On a concentric perceptive field explanation of the illusion, these results suggest that a broad distribution of perceptive field sizes exists at each visual field location. The peak of this distribution lies close to the previously reported perceptive field size at that location. As the adaptation level decreases, the distribution shifts upwards in size. At scotopic levels lateral inhibition only occurs for large stimuli. The size distribution can be used to account for the spatial extent of some contrast phenomena.

Adaptation, Ocular↗