Topographic contribution of early visual cortex to short-term memory consolidation: a transcranial magnetic stimulation study

Abstract

The neural correlates for retention of visual information in visual short-term memory are considered separate from those of sensory encoding. However, recent findings suggest that sensory areas may play a role also in short-term memory. We investigated the functional relevance, spatial specificity, and temporal characteristics of human early visual cortex in the consolidation of capacity-limited topographic visual memory using transcranial magnetic stimulation (TMS). Topographically specific TMS pulses were delivered over lateralized occipital cortex at 100, 200, or 400 ms into the retention phase of a modified change detection task with low or high memory loads. For the high but not the low memory load, we found decreased memory performance for memory trials in the visual field contralateral, but not ipsilateral to the side of TMS, when pulses were delivered at 200 ms into the retention interval. A behavioral version of the TMS experiment, in which a distractor stimulus (memory mask) replaced the TMS pulses, further corroborated these findings. Our findings suggest that retinotopic visual cortex contributes to the short-term consolidation of topographic visual memory during early stages of the retention of visual information. Further, TMS-induced interference decreased the strength (amplitude) of the memory representation, which most strongly affected the high memory load trials.

Figure 1.

Experimental design. The memory task was a modified version of the change detection task that contained a sample presentation phase, a retention phase and a test phase. Top left, Sample and test stimulus arrays contained sample and test arrays of one (low memory load) or three (high load) simultaneously presented abstract shapes, which were always presented at either the left or the right side of the fixation point, at 6° visual angle, and dispersed at equal distances of 20° polar angle steps. Right, A memory trial started with a short presentation of a visual field cue (< or >) that designated the visual field in which the trial would be presented (cue validity = 100%). Afterward, the sample array was briefly shown and was followed by a retention interval of fixed duration (1500 ms). Finally, the retention interval was followed by presentation of the test array and participants were required to judge whether the test array was the same as the sample array (i.e., match) or not (nonmatch). For TMS trials, a single TMS pulse was delivered at 100, 200, or 400 ms into the retention interval (ISI). For baseline measurements, no pulses were delivered. Bottom left, TMS pulses were delivered over lateral occipital cortex, using the ipsilateral visual field as TMS-naive control. LVF, Left visual field.

Figure 2.

Pilot results. Plots show accuracy (A′) of memory trials with memory masks at 100, 200, or 400 ms into the retention interval. Here, memory masks were always spatially congruent to the memory trial. A, Mean accuracies are shown for the two memory loads (solid lines, load 1; dashed lines, load 3) and for the two visual fields [red, left visual field (LVF); blue, right visual field (RVF)]. ANOVA showed that responses were similar across the two visual fields. B, Mean accuracies are shown for the two memory loads collapsed across visual fields. Error bars represent 1 SEM.

Figure 3.

TMS and memory-masking results. A–D, Plots show accuracy (A′) of memory trials with TMS pulses (A, B) or memory masks (C, D) at 100, 200, or 400 ms into the retention interval. Here, TMS pulses and memory masks were spatially congruent to the memory trial in 50% of the trials. A, C, Mean accuracies are shown for the two memory loads and two visual fields after presentation of a TMS pulse (A) or memory mask (C). B, D, Mean interference effect of accuracy after presentation of a TMS pulse (B, TMS visual field—Control visual field) or a memory mask (D, Mask visual field—Control visual field). Error bars represent 1 SEM. TMS/Mask, TMS or memory-mask visual field; Ctrl, control visual field.