SPATIOTEMPORAL IMAGING OF CORTICAL ACTIVATIONS DURING VERBAL MEMORY ENCODING TASKS

J. Lian¹, A. Goldstein², E. Donchin³, B. He¹
¹Department of Bioengineering, University of Illinois at Chicago, USA
²Department of Psychology, Bar-Ilan University, Israel
³Department of Psychology, University of Illinois at Urbana-Champaign, USA

Abstract: The spatiotemporal imaging of cortical activations during memory encoding was conducted by analyzing event-related potentials (ERPs) recorded from 11 healthy subjects who participated in word encoding tasks using rote strategy. Cortical potentials were imaged noninvasively from scalp ERPs. Different levels of brain activation were found in the left inferior prefrontal, left medial temporal and left parietal lobes with different latencies after onset of event. It is concluded that these regions work jointly across both spatial and temporal domains to promote verbal memory formation.

INTRODUCTION

Previous findings suggest that different neural-systems participate in the encoding of episodic memory. It has been reported that the left prefrontal cortex underpins the beneficial effects of semantic processing on the subsequent memory, while the right prefrontal cortex seems more associated with the pictorial events processing [1-2]. In addition, the medial temporal lobe (MTL) has also been suggested to contribute to episodic memory formation [1-3].

The event related potential (ERP) has excellent temporal resolution desired for memory study. Different ERP components have been correlated with subsequent memory. Particularly, the P300, which is elicited by deviant events presented within the context of the oddball paradigm [4,5], has been demonstrated to be positively correlated with the subsequent recall performance if rote rehearsal strategy is used. However, the scalp ERP has limited spatial resolution due to the head volume conduction effect [6], thus could not provide spatial details regarding the underlying sources. Much effort has been made during the past decades for high-resolution imaging of brain sources from scalp potentials [7-12]. Of particular interest is the cortical imaging technique (CIT) [10-12], which provides an estimate of cortical potentials (CPs) that would have been recorded had the electrodes been placed directly over the epicortical surface.

In the present study, we conducted CIT analysis for spatiotemporal mapping of brain dynamic activations during verbal memory encoding tasks when using rote strategy.

METHODS

Eleven right-handed subjects who gave informed consent were enrolled in the present study under a protocol approved by the IRB/UIUC. Each participant studied 40 lists of 15 words each for subsequent memory testing. The lists were presented one word at a time on a monitor. Each list contained one isolate word with larger font randomly assigned in the 6^th-10^th position. The non-isolate words within these positions were considered control conditions. Participants were instructed to memorize the words using rote strategy, i.e., by silently repeating the words to themselves. Right after the presentation of each list, the participant’s memory for the word list was tested by a pencil-and-paper free recall test. The scalp potential was recorded during the presentation of the word lists with 129-channel EGI system using Cz as reference. Data were then re-referenced to linked-mastoids, and corrected for eye movement. ERPs were obtained by averaging trials of each condition for each subject, and were digitally bandpass filtered and baseline corrected. For each of the 4 conditions (recalled or not recalled, for the isolates and control words), ERPs were grand averaged across all 11 subjects for CIT analysis. The CIT method used in the present study is based on a concentric 3-sphere inhomogeneous head model [10]. A closed-surface dipole layer was constructed to equivalently represent all the enclosed brain sources. The equivalent dipole layer strength was first estimated from the scalp potentials by solving the inverse problem, then the CP maps were reconstructed by solving the forward problem, from the estimated equivalent diple layer to the CPs.

RESULTS

Figure 1 shows the midline 3-channel ERP waveforms for the isolates and controls, separated as a function of whether they were or were not subsequently recalled. Clearly, isolates elicited larger N400 and P300 than the controls.

Figure 1. The ERP waveforms in three midline electrodes elicited by the isolates (left) and the controls (right).

Figure 2 displays the CP maps (top view, nasion upward) elicited by the isolates at 4 time instants, corresponding to (a) not-recalled and (b) recalled isolates. Multiple areas of brain activation were observed, including (A) left inferior prefrontal lobe (IPFL), (B) left MTL, (C) left parietal lobe (PL), (D) right prefrontal lobe, (E) right central lobe, and (F) right occipital lobe. Comparing the recalled with the not-recalled CP maps, three regions of interest (ROIs) show stronger activation: (A) the left IPFL during P200 and P300, (B) the left MTL and (C) the left PL at all time instances. Similar results were obtained for the controls, except that both left IPFL and left MTL show enhanced activation at 400 ms and thereafter for the recalled words.

Figure 2. Cortical potential maps at 4 time points corresponding to (a) not-recalled and (b) recalled isolates.

To quantify the spatio-temporal potential difference, Figure 3 compares the local maximal CP variation with time in the three ROIs, for the recalled and not recalled isolates. Clearly, isolates elicited stronger left IPFL activation (200 ms and 500 ms), followed by stronger left MTL activation (500 ms and slow wave), together with persistent stronger left PL activation, for the recalled than those not-recalled isolates.

Figure 3. Local maximal cortical potential variation with time in the three ROIs, for the recalled and not recalled isolates.

DISCUSSION

Consistent with previous findings [1-3], the present study shows that stronger activation of the left IPFL and the left MTL is associated with the subsequent memory formation. Furthermore, our results indicate that stronger activation of the left PL is also highly correlated with the subsequent memory. It has been suggested that there might exist temporal relationship between the frontal and MTL regions during memory encoding, with the frontal lobe provide input to the MTL region for information integration and memory formation [13-14]. Our findings for the first time, provide compelling evidence supporting this hypothesis. The preceding left IPFL activation and the subsequent left MTL activation operate serially to support effect memory encoding.

ACKNOWLEDGEMENT

This work was supported in part by NSF CAREER Award BES-9875344 and NIH-MH19554.

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