Pinpointing the neural signatures of single-exposure visual recognition memory

Memories of the images that we have seen are thought to be reflected in the reduction of neural responses in high-level visual areas such as inferotemporal (IT) cortex, a phenomenon known as repetition suppression (RS). We challenged this hypothesis with a task that required rhesus monkeys to report whether images were novel or repeated while ignoring variations in contrast, a stimulus attribute that is also known to modulate the overall IT response. The monkeys’ behavior was largely contrast-invariant, contrary to the predictions of an RS-inspired decoder, which could not distinguish responses to images that are repeated from those that are of lower contrast. However, the monkeys’ behavioral patterns were well-predicted by a linearly decodable variant in which the total spike count was corrected for contrast modulation. These results suggest that the IT neural activity pattern that best aligns with single-exposure visual recognition memory behavior is not RS but rather “sensory referenced suppression (SRS)”: reductions in IT population response magnitude, corrected for sensory modulation.

人们认为，我们所见过的图像记忆，会体现在颞下（inferotemporal, IT）皮层等高级视觉脑区的神经响应衰减现象中，该现象被称为重复抑制（repetition suppression, RS）。我们通过一项实验任务对这一假说提出质疑：实验要求恒河猴判断图像属于新颖还是重复呈现，同时忽略对比度的变化——对比度作为一种刺激属性，已知同样会调控颞下皮层的整体响应。恒河猴的行为表现基本不受对比度影响，这与基于重复抑制的解码器的预测结果相悖：这类解码器无法区分对重复呈现图像的响应与低对比度图像的响应之间的差异。不过，采用一种可线性解码的修正模型时，我们可以精准预测恒河猴的行为模式：该模型针对对比度调控对总脉冲计数进行了校正。上述结果表明，与单次视觉暴露后的识别记忆行为最契合的颞下皮层神经活动模式，并非重复抑制，而是“感官参照抑制（sensory referenced suppression, SRS）”：即经感官调控校正后的颞下群体响应幅度衰减。