Pupil response as a predictor of blindsight in hemianopia

Abstract

Significantly above-chance detection of stimuli presented within the field defect of patients with postgeniculate lesions is termed "blindsight." It has been proposed that those with blindsight are more likely to benefit from visual rehabilitation by repeated stimulation, leading to increased visual sensitivity within their field defect. Establishing the incidence of blindsight and developing an objective and reliable method for its detection are of great interest. Sudden onsets of a grating pattern in the absence of any change in light flux result in a transient constriction of the pupil, termed "pupil grating response." The existence of pupil grating responses for stimuli presented within the blindfield has previously been reported in a hemianopic patient and two monkeys with removal of the primary visual cortex unilaterally. Here, we have systematically investigated the presence of a spatial channel of processing at a range of spatial frequencies using a psychophysical forced-choice technique and obtained the corresponding pupil responses in the blindfield of 19 hemianopic patients. In addition, in 13 cases we determined the pupil responses in a sighted field location that matched the blindfield eccentricities. Our findings demonstrate that blindfield pupil responses are similar to those for the sighted field, but attenuated in amplitude. Pupillometry correctly characterized the presence or absence of a significant psychophysical response and thus is worth measuring in the cortically blindfields as a predictor of intact psychophysical capacity. The incidence of blindsight where detection performance had been investigated psychophysically over a range of spatial frequencies was 70%.

Keywords: incidence; screening; spatial vision.

Fig. 1.

Brief summary of details on patients participating in this study. Details include age at the onset of injury, when tested in the laboratory, sex, and the type of brain injury as well as a schematic representation of the binocular visual fields based on monocular results. Areas of reduced sensitivity in one or both eyes are shown in gray. Black areas are those where sensitivity of 0 db is measured in both eyes. The target locations for blind and sighted stimulus presentations are also depicted.

Fig. 2.

The response profile for the psychophysically determined spatial channel of processing is shown in green lines and symbols (triangles). Pupil responses in the blind and sighted fields are plotted in pink (squares) and blue (diamonds). The last two panels (Bottom Right) show the average responses. In all panels, the spatial frequency in cycles per degree is plotted along the horizontal axis; the left vertical axis indicates the pupil response amplitudes in mm; the right vertical axis indicates the percentage of correct responses in a two-alternative forced-choice psychophysical detection task.

Fig. 3.

(A) Response matrix summarizing the presence or the absence of significant (P < 0.05) psychophysical and pupil responses in all 20 cases. (B) The relationship between the amplitude of psychophysical and pupil responses is explored by converting the response amplitudes to z-scores and plotting them against each other. The plot shows no significant correlation between these measures of effect size.

Fig. 4.

A graphical representation of a method for detection of pupil response. A typical average pupil response is shown in pink. The time interval of interest is between 250 and 750 ms poststimulus onset. The first derivative (blue line) and three-sample average (gray) lines are also shown together with threshold limits (yellow lines). The response onset is when the first derivative is negative and the three-sample average is lower than the threshold value (0.1 mm/s). Response plateaus when both the first derivative and the three-sample average are zero or positive. The difference between the pupil diameters averaged over six samples immediately before the response onset and after the response offsets reflects the mean response amplitude.