146

repeated after 24 or 48 h, 'old' (already used) words were identified faster than newly added ones.

Prominent components of the ERP were N1 /P2 , P300, and LPC. Mainly the N 1 / P 2 and LPC varied as a function of conditions and between trials, i.e., successive presentat ions of each stimulus. ERP parameters indicate that both tonic and phasic changes of threshold element properties may modulate retrieval from LTM and stimulus identification already at early, preconscious stages of processing.

STUDIES OF R E C O G N I T I O N M E M O R Y WITH EVENT- R E L A T E D BRAIN POTENTIALS

Michael Rugg Wellcome Brain Research Group, University of St. Andrews, St. Andrews, U K

Event-related brain potentials (ERPs) are an attractive means of studying brain activity evoked during memory tasks because of their non-invasive recording methodology, their good time resolution and the fact that they can provide an indirect measure of the processing of stimuli to which no overt re- sponse is made. Furthermore, as the intracerebral generators of memory-related ERP effects become known, ERPs may offer a means of studying the time-course and interaction of specific neural systems implicated in normal and abnormal memory. This presentat ion will review recent work on recog- nition memory for words and pictures in normal and brain-le- sioned populations, carried out using both 'direct ' and 'indi- rect' memory tasks. ERP evidence for the fractionation of recognition memory into multiple, interacting processes, will be presented. The relevance of this research for current models of recognition memory will be discussed.

P3 latency was reported as correlating with st imulus evalua- tion time, reflecting processes of memory scanning during short-term memory tasks. It has been suggested that P3 ampli- tude indicates updating processes of the working memory triggered by P3 eliciting processes. Utilizing a modified ver- sion of the Sternberg probe identification task, ERPs recorded during probe identification were investigated for influences from the preceding probe identification processes. The probes (letters) were classified into two categories according to the shape of the letters. The sequences of consecutively-presented probes were varied between the two categories. The probabil- ity of single probes and probe sequences occurring was the same in each trial (multiple tests of whether one set of items had been memorized) and over the session. Processes of activation and preparedness were controlled by introducing the probe presentation as $2 within a CNV paradigm. Se- quences were combined for 'change ' of categories and 'no change ' of categories prior to analysis. ERPs were calculated for 'change ' and 'no change' sequences, each of which was in turn separated into positive probes (items in the list to be memorized) and negative probes (items not in the list). Reac- tion time measures were statistically analyzed for the same conditions. The subjects were 14 students. EEG was recorded from 11 leads, data are presented for Pz.

In addition to the well-known effects of both setsize and negative probes on the P3 latency results, decreased P3 posi- tivities and delayed latencies for the 'change ' -sequences were observed for positive probes only, on the P < 0.01 level. Apart from increasing reaction times with increasing setsize, there were no effects on reaction time measures. The results of P3 amplitudes and latencies are interpreted as being correlates of memory updating processes caused by the presentat ion of positive probe stimuli, rather than being due to excess activa- tion correction.

SPACE-TIME S T R U C T U R E OF SELECTIVE VISUAL A T T E N T I O N

E. Ruhnau and V. Haase Institute of Medical Psychology, Miinchen, Germany

Saccadic eye movements directed either to auditory or visual targets are investigated. Concerning spatial properties, it is shown that saccadic latencies for mirror-symmetric target positions are correlated in the visual modality, but not in the auditory one. Concerning temporal properties, it is shown that the attentional shift between different spatial positions and different modalities is based on discrete temporal processing of high frequency (approx. 25 Hz).

M E M O R Y U P D A T I N G EFFECTS ON ERPs D U R I N G A M E M O R Y SCANNING TASK

G. Sammer Institute of Psychology, University of Hamburg, Hamburg, Germany

SPECIFIC DISORDERS OF KNOWLEDGE: ANATOMI- CAL BASIS

Giuseppe Sartori Universit~ di Padova, Padua, Italy

Recently, a number of investigators have pointed out that disorders of knowledge may be observed in absence of other cognitive deficits after focal brain lesions. Patients with disor- ders of knowledge show a degradation of information, both perceptual and non-perceptual which may be pinpointed in a number of different tasks, such as naming of visually-pre- sented objects, naming on verbal description, verbal descrip- tion of concepts, drawing from memory, etc. These patients, in other terms, fail to recognize objects because they have lost the knowledge about objects but their pre-semantic percep- tual processes may be shown to be normal. Such a pattern is different both with respect to apperceptive agnosia and asso- ciative agnosia.

In this paper, a review of 10 published and 5 unpublished cases on which in-depth single case investigation proved a