Acta Psychologica 37 (1973) 279-282.0 North-Holland Publishing Company 1973

TIME REPRODUCTIONS BY H.M. 1

Whitman RICHARDS

Depa?tment of Psychology, Massachusetts Institute of Technology, Cambridge, Mass. 02139

Time reproductions by a subject with a severe memory loss were normal only for intervals less than 20 sec. For longer intervals, the reproduced intervals were roughly proportional to the square root of the standard intervals, plus 20 sec.

H.M., the justly famous case of Scoville and Milner (1957), is unique because of his severe memory loss (Milner 1962; Milner et al. 1968). This individual, who is now over 40 years old, fails to recognize a person he has met 30 minutes before and with whom he may have conversed for 5 or 10 minutes. His recollection of statistical facts and figures is over 15 years out of date. Only a sprinkling of new informa- tion and assorted events have been added to H.M.‘s long-term memory since the time he underwent a bilateral surgical removal of cortex from the medial temporal zones, for relief of severe epileptic seizures (Scoville and Milner 1957).

Without the normal recall for events, how fast does time pass for H.M.? Does one hour, one day or one year seem just as long to this unique individual as to us?

To provide a partial answer to this question, H.M. was asked to reproduce time intervals ranging from 1 to 300 sec. This method of generating time estimates appears the least susceptible to undesirable effects of spacing, order, range and extraneous stimuli (Richards 1964).

‘This study was initiated at the suggestion of Drs. B. Mihrer and H.-L. Teuber, who also helped to provide the opportunity to work with H.M. under a collaborative arrangement with H.M.‘s physician, Dr. W.B. Scoville. Financial support was received by grants to Professor Teuber from the John A. Hartford Foundation, and NIMH (under MH-05673 and FR88 to the M.I.T. Clinical Research Center).

280 W. Richards, Time reproductions by H.M.

The procedure is very simple: the experimenter presents an interval marked by verbal “go” and “stop“ commands. Following presentation of the interval, the subject, by giving similar “go” and “stop” commands, attempts to reproduce the same intervaL All measurements were made with the second hand of a stopwatch. The estimated accuracy of the measurement was about to.2 sec. Except for the shortest interval, this error is negljgible compared with the variability of the individual’s reproductions.

All intervals were measured in a quiet room without distractions. There was no conversation during the measurements, for chatting caused the subject to “forget” both the task and the

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Fig. 1. Time intervals reproduced by H.M. at two different sessions which were one year apart. If reproduced time equals true time, the points would lie along the 45” diagonal. The histo- grams show the distribution of the individual measurements for each standard interval. TWO entirely different power functions are needed to fit the data.

W. Richards, Time reproductions by H.&l. 281

interval to be reproduced (unless very short). As a further aid, a printed card was used to indicate whether the standard or the reproduced intervals were in progress. This sign also included the note “Please do not count!“, to remind the subject not to consciously “count- out” the time intervals. (This helpful reminder card was suggested by Dr. Suzanne Corkin.)

The results of two sets of measurements, taken one year apart, are given in fii. 1. The axes on this figure are logarithmic. Thus, if reproduced time approximately equals true time, then the data should he on a line near 45”, as they do for normal subjects (Fraisse 1963; Richards 1964). Only for time intervals less than 20 set do H.M.‘s data fall very close to the 45” diagonal. For intervals longer than 20 set, however, the measurements fall well away from the 45” diagonal. The power function describing this portion of H.M.‘s data has an exponent of 0.44 - an abnormally low value.

The distinct transition in the nature of the measurements at 20 set suggests that two entirely different mechanisms may be operating at short and long intervals. For intervals less than 20 set, the timing mech- anism of H.M. appears normal. His decay of short-term memory is also normal in this region (Wickelgren 1968). However, for intervals exceed- ing 20 set, the “clock” used by the normal subject seems to be absent in H.M., or, at best, is markedly upset. The transition from the normal to the abnormal behavior of time reproductions for intervals greater than 20 set suggests a failure in a separate longer-term temporal inte- grator or, alternatively, an inability to accurately recycle (or count) outputs from a shorter-term integrator. Either possibility may be con- sidered equivalent to a kind of memory deficit, although the nature of the deficit would obviously be quite different in each case. As such, the results do not differentiate between either a “sensory” or a “cognitive” model of the time sense (see Vroon 1972).

With the data of fig. 1 we can now provide a qualified answer to the question of how long one hour or one day may appear to H.M.. Assum- ing that the curve in fig. 1 can be extrapolated, then one hour to us is like 3 minutes to H.M.; one day is like 15 minutes; and one year is equivalent to 3 hours for H.M.. Using such a revised time-scale. H.M.‘s memory span might be normalized to ours.

References

Fraisse, P., 1963. The psychology of time. New York: Harper & Row, p. 213. Milner, B., 1962. Les troubles de la memoire accompagnant des lesions Hippocampiques bilater-

ales. In: Physiologie de l’hippocampe. Paris: Centre National de la Recherche Scientifique, pp. 251-212.

Milner, B., S. Corkin and H.-L. Teuber, 1968. Further analysis of the hippocampal amnesic syndrome: 14-year follow-up study of H.M. Neuropsychologia 6,215-234.

282 W, Richards, Time reproductions by H.M.

Richards, W., 1964. Time estimates measured by reproduction. Perceptual and Motor Skills 18, 929-943.

Scoville, W.B. and B. Mihrer, 1957. Loss of recent memory after bilateral hippocampal lesions. Journal of Neurology, Neurosurgery and Psychiatry 20, 11-21.

Vroon, P.A. and A. van Boxtel, 1972. Testing some implications of the sensory physiological model of the time sense. Psychologische Forschung 35,8 l-92.

Wickelgren, W.A., 1968. Sparing of short-term memory in au amnesic patient: implications for strength theory of memory. Neuropsychologia 6, 235-244.