Thom Carney

Pupil dilation during visual target detection

It has long been documented that emotional and sensory events elicit a pupillary dilation. Is the pupil response a reliable marker of a visual detection event while viewing complex imagery? In two experiments where viewers were asked to report the presence of a visual target during rapid serial visual presentation (RSVP), pupil dilation was significantly associated with target detection. The amplitude of the dilation depended on the frequency of targets and the time of target presentation relative to the start of the trial. Larger dilations were associated with trials having fewer targets and with targets viewed earlier in the run. We found that dilation was influenced by, but not dependent on, the requirement of a button press. Interestingly, we also found that dilation occurred when viewers fixated a target but did not report seeing it. We will briefly discuss the role of noradrenaline in mediating these pupil behaviors.

Electrophysiological estimate of human cortical magnification

OBJECTIVE The cortical magnification factor characterizes the area of human primary visual cortex activated by a stimulus as a function of angular distance from an observers line of sight. This study estimates human cortical magnification using an electrophysiological method with excellent temporal resolution: visual evoked potential (VEP) dipole source localization. METHODS For each of 60 independently modulated checkerboard patches within the central 18 deg of the visual field, location, orientation, magnitude, and time-course of the dipole current source that best described the VEP distribution across a multi-electrode array was obtained. At numerous eccentricities, cortical magnification was determined using two different techniques: (1) the distance between each pair of adjacent stimulus patches was matched to the corresponding distance between adjacent cortical sources; and (2) the area of each stimulus patch was matched to the magnitude of the corresponding cortical source (which was assumed to be proportional to cortical area). RESULTS The estimates of human cortical magnification using our electrophysiological method were similar to previous estimates from psychophysics, cortical stimulation, and functional magnetic resonance imaging. CONCLUSIONS The concordance of results provided by these disparate technologies, with differing spatial and temporal limitations, supports their combination in studying the spatio-temporal dynamics of human brain function.

Orientation, masking, and vernier acuity for line targets

In an attempt to uncover the properties of the psychophysical spatial mechanisms which optimally respond to the vernier offset between two abutting lines, we investigated the effects of one-dimensional band-limited spatial noise masks superimposed with the target, on vernier thresholds. Unidirectional vernier thresholds were measured in the presence of masks varying in orientation, spatial frequency content and luminance modulation. Because of the dependence of vernier thresholds on target visibility, the effects of these masks on target detection thresholds were also measured. In accordance with the results of Findlay [(1973) Nature, 241, 135-137] but contrary to an hypothesis that the direction of the vernier offset is mediated by the differential output of spatial filters of a single orientation, our results reveal a bimodal orientation tuning function for vernier acuity. We propose that, for offset line targets, the differential responses of at least two filters with orientations which straddle the target lines are combined to extract relative position information. The spatial frequency tuning characteristics of the optimal mechanisms for mediating vernier information are similar to those optimal for detecting the target lines themselves, except that they are tuned to a slightly higher spatial frequency and have a slightly narrower bandwidth. The spatial mechanisms most sensitive to the vernier offset and to target detection exhibit similar responses to increases in mask modulation. This finding suggests that these tasks are limited by the same source of noise, and explains why under a variety of experimental manipulations, equally visible vernier targets result in similar vernier thresholds.

Vernier acuity as line and dipole detection.

The vernier judgment is commonly thought of as discriminating the displacement of a portion of a pattern. However, we have found it revealing to consider vernier stimuli in another light; as the composite of a test pattern superimposed on a masking pedestal. The pedestal is the pattern with zero spatial offset, and the test pattern is the luminance distribution which, when added to the pedestal, produces the offset. For example, a vernier offset of an edge can be generated by adding a thin line (the derivative of an edge) to one half of an edge pedestal, and a vernier offset of a line can be generated by adding a thin dipole (the derivative of a line) to one half of a line pedestal. Vernier thresholds for low contrast edge and line pedestals can be directly predicted from detection thresholds of thin lines and dipoles on uniform fields. A surprisingly simple relationship is also derived between vernier thresholds and the size of Riccos integration zone. We have found this masking paradigm to be fruitful and believe it is relevant to all the hyperacuities, not just vernier.

A physiological correlate of the pulfrich effect in cortical neurons of the cat

When a swinging pendulum is viewed with a light-attenuating filter before one eye, the pendulum bob is perceived to move in an elliptical path in depth. It is believed that the filter causes this illusion, the Pulfrich effect, by delaying processing of the image in the filtered eye relative to that of the unfiltered eye. We sought a physiological correlate of this effect by studying binocular integration in cortical neurons of cats while they viewed moving stimuli. Special attention was focused on single unit disparity tuning because it is widely believed that depth perception is related to the responses of disparity selective neurons in visual cortex. We found that placing a filter before one of the cats eyes produced a temporal delay in the cortical response. The temporal delay was always associated with a shift in the neurons spatial disparity tuning. The observed temporal delays and disparity shifts are comparable with the magnitude of the Pulfrich effect in humans.

Orientation discrimination as a function of stimulus eccentricity and size: Nasal/temporal retinal asymmetry

Orientation discrimination threshold is a monotonically increasing function of retinal eccentricity. Increasing stimulus length extends the range of eccentricities over which fine orientation discriminations can be made. Orientation discrimination thresholds at all eccentricities are determined by the size of the cortical image of the stimulus. Thresholds obtained using either nasal or temporal hemiretina are similar up to the blind spot, beyond which the temporal retina yields increasingly higher thresholds. The results are consistent with a recent theoretical study which predicts that orientation discrimination threshold is determined by the number of cortical cells activated by the discrimination target.

The adjacent pixel nonlinearity: problems and solutions.

The adjacent pixel nonlinearity refers to the dependence of the luminance of a given pixel on the preceding pixel or pixels. We measured this nonlinearity for two CRT displays by measuring the average luminances of a variety of test patterns with different luminance jumps. A two-stage model proposed by Mulligan and Stone was used to fit the data [Mulligan, J.B. & Stone, L. S., (1989). Journal of the Optical Society of America A, 6, 1217-1227 (1989)]. The results show that the model predicts our data well. Based on our measurements and the modeling results, a double-entry look-up table was created to compensate for this nonlinearity. This compensation method works even if the current pixel depends on more than one preceding pixel. Observers commented that at small pixel sizes the compensation results in a sharp, accurate image. Advantages and problems of this compensation will be discussed.

Modelfest: year one results and plans for future years

A robust model of the human visual system (HVS) would have a major practical impact on the difficult technological problems of transmitting and storing digital images. Although most HVS models exhibit similarities, they may have significant differences in predicting performance. Different HVS models are rarely compared using the same set of psychophysical measurements, so their relative efficacy is unclear. The Modelfest organization was formed to solve this problem and accelerate the development of robust new models of human vision. Members of Modelfest have gathered psychophysical threshold data on the year one stimuli described at last years SPIE meeting. Modelfest is an exciting new approach to modeling involving the sharing of resources, learning from each others modeling successes and providing a method to cross-validate proposed HVS models. The purpose of this presentation is to invite the Electronic Imaging community to participate in this effort and inform them of the developing database, which is available to all researchers interested in modeling human vision. In future years, the database will be extended to other domains such as visual masking, and temporal processing. This Modelfest progress report summarizes the stimulus definitions and data collection methods used, but focuses on the results of the phase one data collection effort. Each of the authors has provided at least one dataset from their respective laboratories. These data and data collected subsequent to the submission of this paper are posted on the WWW for further analysis and future modeling efforts.

Development of an image/threshold database for designing and testing human vision models

Models that predict human performance on narrow classes of visual stimuli abound in the vision science literature. However, the vision and the applied imaging communities need robust general-purpose, rather than narrow, computational human visual system models to evaluate image fidelity and quality and ultimately improve imaging algorithms. Psychophysical measure of image imaging algorithms. Psychophysical measures of image quality are too costly and time consuming to gather to evaluate the impact each algorithm modification might have on image quality.

Vernier acuity during image rotation and translation: Visual performance limits

Our capacity to detect spatial misalignments a fraction of the distance between retinal receptors in the presence of image motion challenges our understanding of spatial vision. We find that vernier acuity, while robust to image translation, rapidly degrades during image rotation. This indicates that orientation is a critical cue utilized by the visual system in vernier acuity tasks. Moreover, vernier acuity is robust to translational motion only at high target strengths. Vernier acuity for translating 3-dot targets over midrange velocities can be predicted from vernier acuity data derived from static targets of different presentation durations. However, the degradation observed at higher velocities is greater than predicted. The high velocity degradation reveals that performance is limited by a 1 msec asynchrony sensitivity. The moving vernier stimulus appears to constitute an optimal configuration for the visual system to achieve a 1 msec asynchrony sensitivity by making use of an orientation cue.