Is the rod visual field temporally homogeneous?
(17/3575)
Cone vision has been shown to be temporally inhomogeneous across the visual field. In the periphery, contrast sensitivity is lower for low temporal frequencies and higher for high temporal frequencies. Here we ask a similar question for rod vision at mesopic luminances. Isolation is obtained by testing a well documented rod monochromat. We show that the rod visual field exhibits only a modest degree of temporal inhomogeneity. (+info)
The effect of spatial frequency adaptation on the latency of spatial contrast detection.
(18/3575)
The effect of spatial frequency adaptation on detection response time was studied using 2-D Gabor functions as stimuli. On the basis of pilot studies, it was expected that reaction time to a given spatial frequency at a low contrast would increase following adaptation to that spatial frequency at a high contrast. Subjects were tested using 2-D Gabor functions that ranged in frequency from 25 to 24 cpd. Subjects' reaction times to the Gabor functions were measured prior to adaptation and after adaptation to a particular spatial frequency. The adapting spatial frequency was either 1, 2, 4, 8, 10, or 16 cpd. The test stimuli were 0.3, 0.5, or 0.7 log units above the unadapted threshold contrast. The subjects adapted to the high contrast test grating for 3 min (80% contrast) and reaction times were again measured in an adapt-test-readapt paradigm. The results showed the greatest increase in reaction time after adaptation when adapting and test spatial frequencies within an octave of the adapting spatial frequency also showed an increase in reaction time but to a lesser extent. Reaction times to gratings with spatial frequencies more distant from the adapting spatial frequency were not significantly affected by the adaptation. The results obtained resemble the tuning curves found for threshold data. Reaction times for stimuli at 0.5 and 0.7 log units above the unadapted threshold were affected less by adaptation than those at 0.3 log units above the unadapted threshold. These results were evaluated in terms of a shifting contrast gain mechanism which may account for both the spatial frequency specific effects of adaptation and the differences found for the different contrast test levels. (+info)
The effects of contrast on perceived depth and depth discrimination.
(19/3575)
The contrast dependence of perceived depth was quantified through a series of depth matching experiments. Perceived depth was found to be a power law function of contrast. In addition, subjects exhibited a large uncrossed depth bias indicating that low contrast test patterns appeared much farther away than high contrast patterns of equal disparity. For disparities in the range of +/- 4.0 arc min, matching disparities for low contrast patterns were shifted in the uncrossed direction by the same amount. In other words, while the magnitude of the uncrossed depth bias is a power law function of contrast, it is constant with respect to disparity. In a second series of experiments, the contrast dependence of stereo increment thresholds was measured. Like perceived depth and stereoacuity, stereo increment thresholds were found to be a power law function of contrast. These results suggest that contrast effects occur at or before the extraction of depth and have implications for the response properties of disparity-selective mechanisms. (+info)
S-cone signals to temporal OFF-channels: asymmetrical connections to postreceptoral chromatic mechanisms.
(20/3575)
Psychophysical tests of S-cone contributions to temporal ON- and OFF-channels were conducted. Detection thresholds for S-cone modulation were measured with two kinds of test stimuli presented on a CRT: a rapid-on sawtooth test and a rapid-off sawtooth test, assumed to be detected differentially by temporal ON- and OFF-channels, respectively. S-cone related ON- and OFF-temporal responses were separated by adapting for 5 min to 1 Hz monochromatic (420, 440, 450, 540, or 650 nm in separate sessions) sawtooth flicker presented in Maxwellian view. Circular test stimuli, with a sawtooth temporal profile and a Gaussian spatial taper, were presented for 1 s in one of four quadrants 1.0 degree from a central fixation point. A four-alternative forced-choice method combined with a double-staircase procedure was used to determine ON- and OFF-thresholds in the same session. Following adaptation, the threshold elevation was greater if the polarity of the test stimulus was the same as the polarity of the sawtooth adaptation flicker, consistent with separate ON- and OFF-responses from S-cones. This asymmetrical pattern was obtained, however, only when the adaptation stimuli appeared blue with a little redness. When the adaptation flicker had a clear reddish hue component, the threshold elevation did not depend on the polarity of the sawtooth test stimuli. These results are consistent with a model in which OFF-signals originating from S cones are maintained by a postreceptoral mechanism signaling redness, but not by a postreceptoral chromatic mechanism signaling blueness. (+info)
Segmentation by color influences responses of motion-sensitive neurons in the cortical middle temporal visual area.
(21/3575)
We previously showed that human subjects are better able to discriminate the direction of a motion signal in dynamic noise when the signal is distinguished (segmented) from the noise by color. This finding suggested a hitherto unexplored avenue of interaction between motion and color pathways in the primate visual system. To examine whether chromatic segmentation exerts a similar influence on cortical neurons that contribute to motion direction discrimination, we have now compared the discriminative capacity of single MT neurons and psychophysical observers viewing motion signals with and without chromatic segmentation. All data were obtained from rhesus monkeys trained to discriminate motion direction in dynamic stimuli containing varying proportions of coherently moving (signal) and randomly moving (noise) dots. We obtained psychophysical and neurophysiological data in the same animals, on the same trials, and using the same visual display. Chromatic segmentation of the signal from the noise enhanced both neuronal and psychophysical sensitivity to the motion signal but had a smaller influence on neuronal than on psychophysical sensitivity. Hence the ratio of neuronal to psychophysical thresholds, one measure of the relation between neuronal and psychophysical performance, depended on chromatic segmentation. Increased neuronal sensitivity to chromatically segmented displays stemmed from larger and less noisy responses to motion in the preferred directions of the neurons, suggesting that specialized mechanisms influence responses in the motion pathway when color segments motion signal in visual scenes. These findings lead us to reevaluate potential mechanisms for pooling of MT responses and the role of MT in motion perception. (+info)
Between-algorithm, between-individual differences in normal perimetric sensitivity: full threshold, FASTPAC, and SITA. Swedish Interactive Threshold algorithm.
(22/3575)
PURPOSE: To determine the between-algorithm differences in perimetric sensitivity for the Swedish Interactive Threshold algorithm (SITA) Standard, SITA Fast, FASTPAC, and Full Threshold algorithms; to determine the between-subject, between-algorithm differences in the magnitude of the normal variation in sensitivity. METHODS: The sample comprised 50 normal subjects (mean age, 52.9 +/- 18.5 years) experienced in automated perimetry. One randomly assigned eye was examined at three visits with Program 30-2 of the Humphrey Field Analyzer (HFA). The first visit was a familiarization session. A two-period crossover design with order randomization within visits was used over the second and third visits. SITA Standard, SITA Fast, and HFA 640 Full Threshold were administered during one visit. FASTPAC and HFA 750 Full Threshold were administered during the remaining visit. RESULTS: Group mean Mean Sensitivity was 0.8 dB higher for SITA Standard than for Full Threshold (P < 0.001) and 1.3 dB higher for SITA Fast than for Full Threshold (P < 0.001). A similar trend was found between SITA and FASTPAC. The group mean Mean Sensitivity for SITA Fast was 0.5 dB higher than for SITA Standard (P < 0.001). The pointwise between-algorithm difference in sensitivity was similar for all algorithms. The pointwise between-algorithm, between-subject variability was lower for SITA. The examination durations for SITA Fast and SITA Standard were half those for FASTPAC and Full Threshold; SITA Fast was 41% that of SITA Standard (P < 0.001). CONCLUSIONS: SITA produced marginally higher mean mean sensitivity compared with that of existing algorithms and markedly reduced examination duration. The reduced between-subject variability of SITA should result in narrower confidence limits for definition of normality. (+info)
Olfactory discrimination ability of human subjects for ten pairs of enantiomers.
(23/3575)
We tested the ability of human subjects to distinguish between enantiomers, i.e. odorants which are identical except for chirality. In a forced-choice triangular test procedure 20 subjects were repeatedly presented with 10 enantiomeric odor pairs and asked to identify the bottle containing the odd stimulus. We found (i) that as a group, the subjects were only able to significantly discriminate the optical isomers of alpha-pinene, carvone and limonene, whereas they failed to distinguish between the (+)- and (-)-forms of menthol, fenchone, rose oxide, camphor, alpha-terpineol, beta-citronellol and 2-butanol; (ii) marked individual differences in discrimination performance, ranging from subjects who were able to significantly discriminate between 6 of the 10 odor pairs to subjects who failed to do so with 9 of the 10 tasks; (iii) that with none of the 10 odor pairs were the antipodes reported to differ significantly in subjective intensity when presented at equal concentrations; and (iv) that error rates were quite stable and did not differ significantly between sessions, and thus, we observed a lack of learning or training effects. Additional tests of the degree of trigeminality and threshold measurements of the optical isomers of alpha-pinene, carvone and limonene suggest that the discriminability of these three enantiomeric odor pairs is indeed due to differences in odor quality. These findings support the assumption that enantioselective molecular odor receptors may only exist for some but not all volatile enantiomers and thus that chiral recognition of odorants may not be a general phenomenon but is restricted to some substances. (+info)
Effects of age on the generalization and incubation of memory in the F344 rat.
(24/3575)
Freezing (immobility) in the presence of aversive stimuli is a species-specific behavior that is used as an operational measure of fear. Conditioning of this response to discrete sensory stimuli and environmental context cues has been used as a tool to study the neuropsychology of memory dynamics and their development over the lifespan. Three age groups of F344 rats (3, 9, and 27 month) received tone-foot shock pairing (or tone only) in a distinctive chamber on two consecutive days. Separate subgroups of rats from each age group were then tested, at retention intervals of 1, 20, 40, or 60 days, for context-mediated fear in the environment in which they were trained, for generalization of the fear response to a novel chamber, and for fear of the tone. Beginning at day 20, the 27-month-old rats exhibited less freezing behavior than did younger rats when tested in the conditioning context. This age difference was a result of freezing behavior becoming progressively stronger with time in the two younger age groups, a phenomenon that has been referred to as memory incubation. Incubation of the contextual fear response was not detected in the old rats. In a novel context, all age groups exhibited significantly more freezing than did control animals. There was also pronounced incubation of this generalized freezing response, and the extent of incubation declined significantly with age. In the novel context, the freezing response to the tone was robust in all age groups and increased over time, in constant proportion to the degree of freezing elicited by the novel context itself, prior to tone onset. The fact that old animals are known to be relatively selectively impaired in forms of memory that depend on a functional hippocampus suggests a possible explanation for the reduced incubation effects seen in old rats; however, whether the increased expression of fear over time is mediated by a hippocampal-dependent memory consolidation process or whether it reflects a generalized increase in the gain of the circuitry mediating the fear response itself, remains to be determined. (+info)