The argument for G urgently requires a plausible link to underlying neurophysiological principles if it is not to remain forever a statistical concept. Jensen argues that the correlation between simple reaction time (SRT) and G may imply a relationship between intelligence and a general property of the brain such as neural transmission time. He bases this argument on the assumption that performing SRT is so elementary and automatic that the task is unlikely to include higher level cognitive mechanisms. I criticize this idea in the light of a fundamental distinction between preparation and action, and conclude that we cannot plausibly think of even SRT as a task that does not call on basic components of intellectual function. G therefore remains a construct that is unsupported by more elementary principles governing brain function.
2. The section entitled "Information Processing and G" examines a number of simple perceptual and other decision tasks -- Jensen terms them elementary cognitive tasks or ECT's -- to gain insight into "the biological substrate of individual differences in mental abilities (particularly its general factor, G)" (p. 205). The most elementary of such tasks is simple reaction time (SRT), in which the observer must respond with a button press as soon as the presence of a target is detected. There is a very small but reliable negative correlation between detection speed or reaction time (RT) and IQ, and the correlation increases modestly as the detection task is made more complex by requiring a choice between two targets and a different response to each of them.
3. Jensen assumes that this correlation must reflect something very general about the nervous system, given his a priori claim that G itself must transcend specific cognitive abilities. In addition, he argues that SRT is so elementary a task that it hardly can be said to "involve anything that could be called thinking, cogitation or problem solving in any meaningful sense of these terms. RT's appears (sic) to reflect activity at a basic neural level that occurs prior to the full activation of consciously guided processes" (p. 225). Thus, we can legitimately ascribe differences in SRT (which in turn correlate with intelligence as measured by G) to individual differences in one or more very primitive, functional properties of the nervous system, like neural transmission time or the fidelity of neural signals. Part of Jensen's argument that SRT has more to do with basic neural events than thinking is his rather extreme view that subjects respond in this task before they are even conscious of the target! Jensen appeals to Libet's research (e.g. 1985) as support for this rather outlandish claim. Libet has attempted to measure the time course of a sensation produced by direct electrical stimulation of the exposed cortex in neurosurgical cases and similar sensations produced via the more usual method of cutaneous sensation (a brief pulse to the hand). He concluded that a substantial amount of time (as much as 500 msec) must elapse before cortical processes yield a conscious experience of touch, even when brain tissue rather than skin is directly stimulated. From this Jensen concludes: "subjects in our RT studies had to be responding before they were consciously aware of the reaction stimulus, because the RTs usually averaged less than 500 msec" (p. 225).
4. The neurophysiological evidence that Jensen draws upon in support of his argument that SRT "appears to reflect activity at a basic neural level that occurs prior to the full activation of consciously guided processes" (p. 225) is highly controversial (see for example, Churchland, 1981; Dennett, 1991; Dennett & Kinsbourne 1995). But the section also includes an interesting reference to the opinion of William James (1894), apparently endorsing the idea that responses in SRT experiments can occur before the mechanisms of conscious awareness. According to James, "The whole succession (of the RT process) is so rapid that perception seems to be retrospective and the time order of events to be read off in memory rather than known at the moment" (p. 88). Is it at least plausible then, notwithstanding the lack of crucial evidence, to maintain that a SRT task has little to do with the intellect, and that the correlation between speeded responding and IQ tests is more likely to reflect elementary neural events than cognition? We should be clear, of course, what we mean by this claim. The speeded response to the target may occur "reflexively", to paraphrase James, and perhaps with the minimum of conscious decision, but we are only talking about a subset of the cognitive events that form the necessary background to the task considered as a whole. The endpoint of the SRT task may have little to do with the intellect, but what of the cognitive work that must be done to place oneself in a suitable state of readiness to perform at peak efficiency? James, a master of task analysis, was unambiguous on this point, and his views are completely at odds with Jensen's notion that SRT is devoid of cognitive content, and must simply reflect a general property of the nervous system. James believed that SRT was reflexive, but distinguished between the act of responding to a target and the state of readiness or "foregoing psychic condition" that was ... "a prerequisite for the reflex action". Thus:
"The preparation of the attention and volition; the expectation of the signal and the readiness of the hand to move, the instant it shall come; the nervous tension in which the subject waits, are all conditions of the formation in him for the time being of a new path or arc of reflex discharge" (p. 89).
5. It would seem, then, that we rely on a panoply of higher cognitive skills to be in an optimum state of readiness to perform SRT tasks. Indeed, the very fact that the response is so rapid and focussed, and the individual is, to use James' piquant metaphor, "tingling with premonitory innervation", depends on the efficacy of abilities like selective attention, the monitoring of expectancies, the preparation of a response, the filtering of extraneous thoughts and sensory distraction, the modulation of internal states on the basis of small changes in performance and so forth. Presumably individual differences in these skills are relevant to the outcome of IQ tests, just as they are crucial to the speed with which SRT is accomplished. What then, remains of the hope that G can be linked to neurophysiological substrates? I would think very little, given our present understanding. Jensen attempts to defend G as a construct that is sufficiently coherent to allow a search for possible brain mechanisms. He does so by pointing in a number of disparate directions, but one quickly becomes entangled in a thicket of hypotheses, none of which seem more plausible or implausible than the next.
6. Thus, besides the speed of neural transmission, we have the periodicity of excitatory potentials (p. 255), the differences in the hierarchical organization of functional units (p. 257), inhibition (p. 257), specific modules like working memory in prefrontal cortex (p. 249) and even the possibility of differences (as yet unfathomed) "at the molecular level of neural activity" (p. 260). Such speculations do little to clarify the mechanism underlying G. I understand that G is a statistical fact emerging out of diverse tests of intellectual ability, but does the concept have any biological relevance? Unless its protagonists can go beyond surmise, the question will always beckon, mockingly.
Churchland, P.S. 1981. On the alleged backwards referral of experiences and its relevance to the mind-body problem. Philosophy of Science, 48, pp. 165-181.
Dennett, D. 1991. Consciousness Explained. Boston: Little, Brown and Company.
Dennett, D.C. & Kinsbourne, M. (1995). Time and the observer: The where and when of consciousness in the brain. Behavioral and Brain Sciences 15 (2): 183-247. http://www.cogsci.soton.ac.uk/bbs/Archive/bbs.dennett.html
Jensen, A.R. (1998). The g factor: The science of mental ability. Westport, CT: Praeger.
Jensen, A. (1999). Precis of: The g Factor: The Science of Mental Ability. PSYCOLOQUY 10(23) ftp://ftp.princeton.edu/pub/harnad/Psycoloquy/1999.volume.10/ psyc.99.10.023.intelligence-g-factor.1.jensen http://www.cogsci.soton.ac.uk/cgi/psyc/newpsy?10.023
James, W. 1950. The Principles of Psychology. New York: Dover.
Libet, B. 1985. Unconscious cerebral initiative and the role of conscious will in voluntary action. Behavioral and Brain Sciences, 8, 529-566.