High-speed memory scanning: a behavioral argument for a serial oscillatory model

In order to account for the memory span [G.A. Miller, The magical number seven, plus minus two: some limits on our capacity for processing information, Psychol. Rev. 63 (1956) 81-97.], the magical number seven, plus minus two, and high-speed scanning in human memory ¿S. Sternberg, High speed scanning in human memory, Science 153 (1966) 652-654., Lisman and collaborators [O. Jensen, J.E. Lisman, An oscillatory short-term memory buffer model can account for data on the Sternberg task, J. Neurosci. 18 (1998) 10688-10699; J.E. Lisman, M.A.P. Idiart, Storage of 7+/-2 short-term memories in oscillatory subcycles, Science 267 (1995), 1512-1515.] proposed an oscillatory short-term memory buffer model. In this neurophysiological model: "a single brain network can separately maintain up to seven memories by a multiplexing mechanism that uses theta and gamma brain oscillations for clocking. A memory is represented by groups of neurons that fire in the same gamma cycle" ¿O. Jensen, J.E. Lisman, An oscillatory short-term memory buffer model can account for data on the Sternberg task, J. Neurosci. 18 (1998) 10688-10699, p. 10688. To test this model, we tried to modify the memory scanning time by shifting the gamma oscillation frequency. To this aim, we replicated the visual short-term memory scanning task ¿S. Sternberg, High speed scanning in human memory, Science 153 (1966) 652-654., and we simultaneously used the protocol that Treisman ¿M. Treisman, A. Faulkner, P.L.N. Naish, D. Brogan, The internal clock: evidence for a temporal oscillator underlying time perception with some estimates of its characteristics frequency, Perception 19 (1990) 705-743. designed to drive, slowing down or speeding up, a temporal oscillator acting in the gamma range ¿J.G.R. Jefferys, R.D. Traub, M.A. Whittington, Neuronal networks for induced "40 Hz rhythms, Trends Neurosci. 19 (1996) 202-208; W. MacKay, Synchronized neuronal oscillations and their role in motor processes, Trends Cog. Sci. 1 (1997) 176-183; M. Treisman, N. Cook, P.L.N. Naish, J.K. MacCrone, The internal clock: electroencephalographic evidence for oscillatory processes underlying time perception, Q. J. Exp. Psychol. 47A (1994) 241-289.. In this protocol, an auditory periodic stimulus (click train) was delivered at various frequencies during the task. The reaction time (RT), the slope, and the intercept of the linear function associating RT to memorized list length showed systematic modulations according to the stimulation frequency. The predicted driving effects due to the click trains were obtained, consisting of localised modulations of performance on the stimulation frequency band. We argue that memory scanning is indeed paced by a temporal oscillator, thus providing behavioral arguments for the serial oscillatory model of Lisman.