Gonzalez, Alexander Memory Lab

Jun 08, 2016 10:05 AM

Alex's Thesis Defense today at 11AM in Allen Aud. in CIS.

In any given day, we encounter events or episodes that are more memorable (better encoded) than others.

The memorability of a given event can be experimentally measured by the degree of difficulty (speed, confidence, accuracy), in which that episode is brought back to mind (retrieval).

 Through lesion and animal model studies we have gained a core understanding of what parts of the brain are involved in these behavioral processes. However, the temporal dynamics and cortical spatial specificity of memory encoding and retrieval mechanisms remain a debated topic in neuroscience. Furthermore, much of our knowledge comes from correlational and indirect measures of brain activity.

In the first part of my talk, we leveraged direct recordings from the human cortical surface to study memory processes at a spatial and temporal resolution that remains rare in humans.

When participants were tasked to recognized previously presented stimuli, we observed a temporal cascade of activity in the left parietal region of the brain.

The intraparietal sulcus (IPS) traverses the parietal lobe and shows greater activity for previously seen items starting ~300ms after probe onset and returns to baseline 200ms before a memory decision is made.

On the other hand, a more dorsal region, the superior parietal lobe (SPL), while initially active for both old and new stimuli, remains active through the behavioral response only for new stimuli.

These observed cortical temporal dynamics advance theoretical models of the different roles of these parietal regions in remembering.

During learning, we find that sustained oscillatory activity is predictive of subsequent memory strength.

Specifically, separable oscillations around 6Hz (theta rhythm) and 10Hz (alpha rhythm) are the most predictive of later remembering.

Moreover, we find that the IPS, thought to be a key member of the top-down attentional network, explains the most variance in predicting future memory strength when compared to surrounding regions.

The second part of the talk focuses on investigating the causal role of oscillatory activity in the cortex for memory formation.

A prevalent theory posits that opposing phases of the theta rhythm should promote states of optimal encoding or retrieval.

However, direct evidence for these theorized effects remains scarce.

In this work, we induce the theta rhythm in the dorsal prefrontal cortex through transcranial alternating current stimulation.

Employing a closed-loop protocol, to be learned stimuli are presented at specific phases of the induced theta oscillation. Memory for the presented stimuli is later probed, and we analyze remembered and forgotten stimuli as a function of encoding stimulation phase.

Consistent with the theory, we find that, in all participants, later remembered vs. forgotten stimuli are associated with opposing phases during learning (180-degree difference).

The nature of the experiment establishes a causal role for theta-phase alignment in effective memory formation. Together, my work advances understanding of the electrophysiology of human episodic memory through direct cortical recordings and electrical stimulation.

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Jan 11, 2016 02:54 PM

See Alex's talk today below.

Gonzalez Abstract: Functional MRI has shown that activity in lateral posterior parietal cortex during the encoding of episodic memories predicts later memory outcomes.

In particular, superior parietal lobe (SPL) shows greater activity during events that are subsequently remembered vs. forgotten, which is thought to reflect the role of top-down attention at encoding.

In this experiment, we used electrocorticography (ECoG) to investigate the temporal dynamics of SPL activity during encoding.

Seven epilepsy patients (5 with left coverage), with grids of electrodes covering the intraparietal sulcus (IPS) and the SPL, performed multiple encoding-retrieval blocks.

At encoding, subjects viewed words (20 per block, 2-7 blocks), deciding whether each was abstract or concrete.

During retrieval, 20 new words were randomly intermixed with the studied words, and subjects indicated whether they recognized each word as â€˜oldâ€™ or â€˜newâ€™.

As a proxy for subsequent memory strength we used the response time for correct trials at retrieval and related it to the power and phase of putative frequency bands of the recorded local field potential (LFP). We find that the neural activity at encoding predicts the response time at retrieval at multiple frequency bands and time windows. This was particularly true in the SPL region. Moreover, the LFP activity during the pre-encoding time period is also predictive of future memory strength. Together, our data elucidates how the neural activity dynamics during encoding in the parietal cortex relate to subsequent memory strength