Short-term monocular patching boosts the patched eye's response in visual cortex

Abstract

Purpose: Several recent studies have demonstrated that following short-term monocular deprivation in normal adults, the patched eye, rather than the unpatched eye, becomes stronger in subsequent binocular viewing. However, little is known about the site and nature of the underlying processes. In this study, we examine the underlying mechanisms by measuring steady-state visual evoked potentials (SSVEPs) as an index of the neural contrast response in early visual areas.

Methods: The experiment consisted of three consecutive stages: a pre-patching EEG recording (14 minutes), a monocular patching stage (2.5 hours) and a post-patching EEG recording (14 minutes; started immediately after the removal of the patch). During the patching stage, a diffuser (transmits light but not pattern) was placed in front of one randomly selected eye. During the EEG recording stage, contrast response functions for each eye were measured.

Results: The neural responses from the patched eye increased after the removal of the patch, whilst the responses from the unpatched eye remained the same. Such phenomena occurred under both monocular and dichoptic viewing conditions.

Conclusions: We interpret this eye dominance plasticity in adult human visual cortex as homeostatic intrinsic plasticity regulated by an increase of contrast-gain in the patched eye.

Keywords: Monocular patching; contrast-gain; eye dominance plasticity; intrinsic plasticity; steady-state visual evoked potentials; visual cortex.

Fig.1

Example stimuli used in experiments. Mask (a) and target (b) were patches of static white noise windowed by a raised cosine envelope. The patches were tiled in a 5×5 grid, surrounded by a series of orthogonal lines to aid binocular fusion. The orientation of the grids was varied randomly from trial to trial to minimise local adaptation.

Fig.2

Contrast response functions at the target and mask frequencies. (a) The procedure of the experiment. Contrast response functions for the target frequency and the mask frequency were tested at four configurations before and after the 2.5-hour patching stage. Panels (b–e) show the results for the target frequency and (e–h) show the results for the mask frequency. The four dichoptic conditions are listed in separate columns, in particular, (b) and (f) refer to the condition in which the previously patched eye saw the target and there was no mask in the unpatched eye; (c) and (g) refer to the condition in which the unpatched eye saw the target and there was no mask in the previously patched eye; (d) and (h) refer to the condition in which the previously patched eye saw the target and the unpatched eye saw the mask and (e) and (i) refer to the condition in which the unpatched eye saw the target and the previously patched eye saw the mask. In each panel, the pre-patching measures are presented as unfilled squares and dashed lines and the post-patching measures are presented as filled circles and solid lines. Error bars give±1 standard error across observers (n = 12).

Fig.3

The dichoptic masking effect on SSVEP amplitudes at the target frequency. Relative SSVEP amplitude (with mask/no mask) plotted against target contrast for the patched eye (a) and unpatched eye (b). Points lower than the middle identity line indicate dichoptic masking effects of the mask on SSVEP amplitudes at the target frequency. Error bars give±1 standard error across observers (n =12).

Fig.4

Change of SSVEP amplitudes after patching. Relative SSVEP amplitude (Post/pre) plotted against target contrast for the four viewing conditions. Points above the middle identity line indicate increasing of response after 2.5-hour patching. Error bars give±1 standard error across observers (n = 12).