I’ve decided to work on and post one (very) rough draft of each subchapter of my PhD thesis at a time, so that I can focus on just one part at a time without being distracted by the rest of it. I also find that I write best when writing for an immediate audience. Please feel free to ask any questions and give comments as I go along. I’ll start with the Table of Contents. As you can see, I’m experimenting with a Socratic/question-based approach to chapter organization:

Tentative title: Reactivation of Visual Representations With Transcranial Magnetic Stimulation

It looks like all the abstracts from the Vision Sciences Society conference we were at a few months ago have now been published in the Journal of Vision. Below is the text of the abstract for the poster I presented. If you have any questions, please leave a comment!

TMS induces detail-rich “instant replays” of natural images

We have previously demonstrated that transcranial magnetic stimulation (TMS) can cause the re-perception of recently presented visual stimuli. Here we find that such replays can be experienced for natural scene stimuli, with a level of detail suggesting low-level rather than iconic representations.

TMS was administered using a Magstim dual-pulse setup sending pulses with 50 ms separation through a figure-8 coil. The coil position over occipital cortex was optimized to elicit vivid flashes of brightness (phosphenes) in a darkened room. We screened subjects to find those that perceived large, bright phosphenes near fixation. To these subjects (N=7), we presented pictures of natural scenes and animals for 100 ms, followed by TMS at various ISIs. Subjects provided verbal descriptions, subjective ratings, and drew figures on the screen.

While TMS in a stable visual environment generally elicits phosphenes that are consistent across trials, colorless, and internally featureless, we found that TMS delivered shortly after image presentation led to the perception of clearly defined forms that varied according to the content of the flashed image.

In this experiment, five out of the seven subjects reported percepts that drew from the preceding images. In the most vivid cases, these would appear to be nearly photographic repetitions in portions of the display. In other cases, subjects would perceive uniformly-filled, phosphene-like figures whose outlines matched, in detail, contours drawn from the preceding image (abstract by Wu et. al. describes double-blind validation of these effects). In early trials, subjects experienced the most vivid replay effects within narrow time windows, which varied from subject to subject between 150-250 ms. With continued stimulation, longer ISIs (as much as one second) became effective.

This study indicates that rich, detailed visual information remains encoded well after visual perception has ended, and that TMS can allow conscious access to these nascent low-level representations.

Also, here’s the abstract for our complementary poster: TMS “instant replay” validated using novel double-blind stimulation technique

After a hiatus of more than a year, I’ve decided to try reactivating this jolly ol’ blog. As before, my plan is to periodically share commentary on various interesting happenings in neuroscience and artificial intelligence.

As for myself, for the past year I’ve been doing research using transcranial magnetic stimulation (TMS) to reactivate recently-viewed visual percepts in occipital cortex. The basis of the work can be found in the PhD thesis of my colleague Daw-An Wu, titled “How perception adheres color to objects and surfaces : studies using visual illusions and transcranial magnetic stimulation.” At some point I’ll elaborate on the directions we’ve been pursuing in the past year, which have had some rather fascinating results.