Seeing through miniature eye movements: A hypothesis

S2 Using this model we tested the hypothesis that the reaching motion of the arm is generated by a stiffening wave resulting from co-activation of the transverse and longitudinal muscles. The results suggest that such a mechanism can produce movements similar to those observed in the actual arm. *Supported by: US. Office of Naval Research. US-Israel Binational Science Foundation and Israel Science Foundation SEEING THROUGH MINIATURE EYE MOVEMENTS: A HYPOTHESES E. Ahissar and A. Arieli Dept. of Neurobiology, Weizmann Institute of Science, Rehovot, Israel The principles that underlie the response properties of cortical ‘simple’ cells seem to be elucidated. However, what is simple in the case of immobilized animals receiving simple stimuli is evidently extremely complicated in the freely-moving animal, where complex images meet the natural retinal ‘turbulence’. Even during fixation images are ever stable on the retina due to the fixational (miniature) eye movements. We suggest, on the grounds of data accumulated over the last decades, that the visual system utilizes these movements for perception, rather than filters out their outcome. Visual images are scanned with different frequencies between, roughly, 1–100 Hz where, according to our hypothesis, higher frequencies resolve finer spatial details. During this scanning, dots or edges produce elongated oriented trajectories on the retina which can optimally drive cortical ‘simple-cells’. The spatial location of single edges is encoded by the phase of retinal activation and movement by the frequency of activation. We suggest that these temporally-encoded retinal signals are decoded by thalamocortical circuits in which thalamic ‘relay’ neurons compare the timing of the retinal input against the timing of cortical intrinsic oscillations. When operating in closed loops, such circuits recode spatial textural information by phase and motion by spike counts f cortical simplecells. According to this hypothesis, the selectivity of cortical simplecells to static line orientations is a by-product of their selectivity to the retinal direction of motion of a single dot. SUBSTANTIAL AUDITORY IMPAIRMENTS IN ADULT DYSLEXICS M. Ahissar1, A. Protopapas2 and M. Merzenich2 1 Dept. of Psychology, The Hebrew University of Jerusalem, Jerusalem; Israel; 2 Scientific Learning, Principles, San-Francisco, CA, USA What is the correlation between reading and acoustical discrimination? To test this, we chose an inclusive approach, applying a broad battery of simple auditory tests on a heterogeneous population of adults. The tested group (n = 50) consisted of good readers, poor readers and compensated dyslexics (as assessed by standard reading tests). Strong correlations were found between current single word reading level and acoustical discriminations. Tone identification and discrimination was substantially impaired for poor readers: the number of trials needed to reliably identify a high (1200 Hz) vs. a low (800 Hz) tone was highly correlated with reading (R 2 > 0.5 !). Particularly hard conditions were those requiring identification, discrimination or detection of a tone followed by another sound within 0–300 ms. Under these conditions poor readers could not discriminate between 900 and 1100 Hz, whereas average and good readers had no difficulty at all! However, increasing either the frequency difference between the 2 tones, or the time interval between each tone and the following noise, enabled discrimination for poor readers too. Impaired performance was mainly expressed for tasks requiring spectral processing, whereas detecting a (temporal) gap in a continuous noise was not impaired. These findings indicate that specific, long lasting, difficulties in reading acquisition are related to specific difficulties in simple, non speech discriminations. Moreover, the pattern of ‘holistic’ listening, whereby

poor readers need larger spectral or temporal differences to resolve the single components, may underlie the difficulty in reading acquisition. SHORT TERM BEHAVIORAL ELECTROPHYSIOLOGICAL AND BIOCHEMICAL CONSEQUENCES OF UNDER WATER STRESS IN THE RAT I. Akirav1, J. Wang1, D. Ben-Shachar2, E. Klein2 and G. RichterLevin1 1 Dept. of Psychology, Haifa University, Haifa, Israel; 2 Research Unit of Psychobiology, Dept. of Psychiatry, Rambam Medical Center and Bruce Rappaport Faculty of Medicine, Technion, Haifa, Israel We found that a short (30 s) under water experience results in both short-term (at 1 h) and longer-term (3 weeks) behavioral alteration in the rat, which may be indicative of stress. Here we focused on the short-term effects of the under-water stress. Rats were trained in the Morris water-maze for a spatial memory task for 5 days to a plateau level of performance. On the 6th day, rats were given a minute to swim (without the platform) and then were held for 30 s under water, or just 1 min to swim. One hour later the rats were tested for their ability to perform the spatial memory task. One group was then taken to be tested also in the elevated plus maze. In a second group tissue was collected from the hippocampus, midbrain and striatum, and the levels of monoamines and their metabolites were assessed, using an HPLC. A third group was subjected to an electrophysiological examination. The rats were anesthetized and prepared for acute recording of field potentials in the dentate gyrus. The ability to induce long-term potentiation was examined, since previous reports indicate that stress reduces the probability of achieving long-term potentiation in the hippocampus. The traumatized rats showed deficits in the performance of the spatial memory task, alterations in midbrain levels of monoamines and reduced ability to be potentiated. These results suggest that the under-water stress may be a useful model in the study of the behavioral, electrophysiological and biochemical consequences of stress. DISCOURSE PROCESSING AND WORKING MEMORY IN ALZHEIMER’S DISEASE A. Almor, D. Kempler, M. MacDonald and E. Andersen Program in Neural, Informational and Behavioral Sciences, University of Southern California Los Angeles, CA, USA A prominent aspect of the speech of patients with Alzheimer’s Disease (AD) is the excessive use of pronouns (e.g., it, he) in comparison to full references (e.g., my son, the teacher). This overuse of pronouns has been previously attributed to word finding difficulties (Kempler, 1995). However, in light of the working memory deficits that are widespread in AD (Baddeley, 1992), the overuse of pronouns may also be attributed to a difficulty in maintaining an active memory representation of referents over time. We report the results from a series of crossmodal naming experiments, in which AD and age-matched healthy subjects listened to an auditory context and then read aloud a visually presented single word continuation. The degree to which the appropriateness of visual continuations affects naming latencies indicates the sensitivity of a subject to the conditions that determine appropriateness. Our experiments show that AD patients’ ability to comprehend pronouns is significantly compromised and that they are better able to access information about a referent when a full reference is used instead of a pronoun. As these experiments did not require word finding, their results cannot be explained on the basis of an impairment in accessing word representations in semantic memory. We discuss the implications of these findings to how language, and in particular discourse, is processed in the brain. Baddeley, A. 1992. Working memory. Science, 255(5044), 556–559 Kempler., D. 1995. Language changes in dementia of the Alzheimer type. in Lubliski R (ed.). Dementia and Communication, Singular Publishing Group, San Diego. 98–114.