Biomedical subjects

Avi Chaudhuri

Publications and source records attributed to Avi Chaudhuri.

8 recordsLinked to original sources

PubMed2003

Face recognition with perspective transformation.

The effect of perspective transformation on transfer of face training was investigated in a yes/no recognition task using face stimuli with 42 degrees, 10 degrees, or no perspective convergence. A strong dependence of recognition performance on the magnitude of perspective transformation was found, with large perspective changes such as from 42 degrees at learning to orthogonal at test producing the strongest impairment and small perspective changes such as from 10 degrees at learning to orthogonal at test the least. In a second experiment, the internal and external features of a face from different perspective convergence were artificially combined to produce identical local features between this composite image and the original but producing an impossible perspective transformation from either. The results of transfer between the composite and untouched images showed face recognition to be strongly affected by local featural similarities and relatively insensitive to global coherence of perspective transformation.

Adult↗

PubMed2003

Face recognition is robust with incongruent image resolution: relationship to security video images.

Identifying a criminal captured on conventional security video typically requires matching poor-quality video footage against a high-quality photograph. The authors examined the consequence of such a large discrepancy in image quality. Recognition and matching performance of this incongruent-quality condition was compared with that of a congruent one, in which a high-quality photograph was reduced to a low-quality video. Recognition memory was little affected by this manipulation, whereas matching performance of the incongruent condition enjoyed occasional advantage. The results show that person identification can tolerate a large discrepancy between image qualities of matching stimuli when one of the images is of poor quality.

Adolescent↗

PubMed2003

Experience-dependent regulation of the zincergic innervation of visual cortex in adult monkeys.

Zinc is packaged in, and released from, a subset of glutamatergic synapses in the mammalian telencephalon where it has been shown to act as a potent neuromodulator. In order to establish the functional role for zincergic neurons in visual cortical function and plasticity we have compared the topographic distribution of zincergic terminals in the primary visual cortex (V1) of normal adult vervet monkeys (Cercopithicus aethiops) to that in monkeys monocularly deprived of visual input for short (24 h) or long (3 months) survival times. In normal animals, staining levels for zinc were highest in layers 1-3, 4b, 5 and 6 and lowest in layers 4a and 4c. The laminar and tangential patterns of zinc staining were complementary to staining patterns demonstrated using cytochrome oxidase (CO) histochemistry. Following 3 months of monocular deprivation by enucleation, levels of zinc staining in layers 3, 4calpha and 6a were heterogeneously reduced, clearly revealing the ocular dominance pattern in V1. When compared with the pattern of CO staining, levels of both CO and zinc were reduced in cortical territory innervated by the enucleated eye. Zinc histochemistry also revealed the ocular dominance pattern after only 24 h of monocular impulse blockade induced by enucleation or intravitreal tetrodotoxin infusion. However, by either means of deprivation for 24 h, levels of zinc were increased in deprived-eye stripes relative to nondeprived-eye stripes. These results indicate that zincergic terminals demarcate distinct compartments in the primate visual cortex. Furthermore, levels of synaptic zinc are rapidly and dynamically regulated, suggesting that zinc and/or zincergic neurons participate in mediating activity-dependent changes in the organization of the adult neocortex.

Animals↗

PubMed2002

Differential induction and decay curves of c-fos and zif268 revealed through dual activity maps.

The use of inducible transcription factors for mapping neural activity is now a common procedure. We have previously developed a double-labelling technique that allows visualization of activated neurons after two different stimulation sequences. The technique exploits the differential time course of mRNA versus protein expression of transcription factors. However, the precise details of the differential time course remained unknown. Here, we provide a complete up- and downregulation profile for both the c-fos and zif268 genes, as determined through combined in situ hybridization and immunocytochemical detection of the mRNA and protein products in primary visual cortex. The data presented here can be used in the design of future studies employing double-label mapping of neural activation following a compound stimulus.

Animals↗

PubMed2002

Reassessing the 3/4 view effect in face recognition.

It is generally accepted that unfamiliar faces are better recognized if presented in 3/4 view. A common interpretation of this result is that the 3/4 view represents a canonical view for faces. This article presents a critical review of this claim. Two kinds of advantage, in which a 3/4 view either generalizes better to a different view or produces better recognition in the same view, are discussed. Our analysis of the literature shows that the first effect almost invariably depended on different amounts of angular rotation that was present between learning and test views. The advantage usually vanished when angular rotation was equalized between conditions. Reports in favor of the second effect are scant and can be countered by studies reporting negative findings. To clarify this ambiguity, we conducted a recognition experiment. Subjects were trained and tested on the same three views (full-face, 3/4 and profile). The results showed no difference between the three view conditions. Our analysis of the literature, along with the new results, shows that the evidence for a 3/4 view advantage in both categories is weak at best. We suggest that a better predictor of performance for recognition in different views is the angular difference between learning and test views. For recognition in the same view, there may be a wide range of views whose effectiveness is comparable to the 3/4 view.

Adolescent↗

PubMed2002

The influence of attention on holistic face encoding.

We examined the influence of attention on the formation of holistic face representations using the composite effect (Perception 16 (1987) 747). In Experiment 1, stimuli composed of a face superimposed on a house were shown during encoding. Subjects delineated either the face or the house, thus manipulating attention away or toward the face. In Experiment 2, an intact face image was presented with letters scrolling from top to bottom. Subjects were asked to either ignore the letters or read them and decipher the words that they formed. Aligned and misaligned composites were shown at testing. Recognition performance was consistently better for misaligned than aligned stimuli, regardless of the allocation of attention during encoding. In Experiment 3, we show that the composite effect can be eliminated by a disruption in holistic processing at the time of encoding. We conclude that holistic encoding is one aspect of face analysis that occurs equally well with or without attention.

Adolescent↗

PubMed2002

Task-dependent transfer of perceptual to memory representations during delayed spatial frequency discrimination.

Discrimination thresholds were obtained using a delayed spatial frequency discrimination task. In Experiment 1, we found that presentation of a mask 3 s before onset of a reference Gabor patch caused a selective, spatial frequency dependent interference in a subsequent discrimination task. However, a 10 s interval abolished this masking effect. In Experiment 2, the mask was associated with a second spatial frequency discrimination task so that a representation of the mask had to be coded into short-term perceptual memory. This experiment was performed to assess whether absence of masking in the 10 s condition of Experiment 1 might be due to decay of the mask information in the perceptual or the memory representational domain. The presence of this second discrimination task now caused similar interference effects on the primary discrimination task at both the 3 s and 10 s interstimulus intervals (ISI) conditions. Finally, to test the robustness of the masking effect, the nature of the secondary masking task was changed from a spatial frequency discrimination task to an orientation discrimination task in Experiment 3. The masking effect was now abolished in both the 3 and 10 s ISI conditions. Together, the results from these experiments are consistent with the idea of a two-level perceptual memory mechanism. The results also suggest that stimulus representations during a perceptual discrimination task are shared between the perceptual and memory representation domains in a task-dependent manner.

Attention↗

PubMed2002

Using MATLAB with the psychophysics toolbox to present the heterochromatic fusion nystagmus stimulus.

We present a program for MATLAB that generates and presents the heterochromatic fusion nystagmus stimulus. This stimulus allows assessment of isoluminant states through recordings of reflexive eye movements (optokinetic nystagmus). The reflexive nature of the subject's response makes this stimulus especially useful with nonverbal subjects, such as children and animals. Unfortunately, the stimulus is complex and difficult to program. By presenting the present program, we hope to help those who wish to use this tool in their research. The code of the function can be downloaded at www. dal.ca/-mcmullen/downloads.html.

Eye Movements↗