on-going cortical activity
The threshold at which a particular fMRI signal is to be accepted for determining the sites over the cortex involved in a neuropsychological test depends on both the signal-to-noise ratio, as well as the amplitude of the signal taken to be of the box-car variety. However, it has been known for some time that the amplitude of the signal, taken as a measure of the transient activity during a neuropsychological test, is conditional on the activity of the part of the cortex of interest at the time of the test: the larger the background signal the smaller the transient signal and vice versa, independent of the neuropsychological test being used.104 Using quantitative relations integrating BOLD responses, energetics and impulse firing, it has been shown how baseline activity can determine the amplitude of both positive and negative sustained BOLD signals.105 It may therefore be necessary to measure the ongoing activity in the area of interest in order to correct for the fMRI amplitude during the neuropsychological test. In animal studies this can be done by using PET to determine the ongoing glucose metabolism, a direct measure of activity, in conjunction with the fMRI measurements.106 This can be carried out using combined MRI/PET, but involves the injection of radioactive fluorodeoxyglucose as a ligand, a procedure that is invasive for human subjects. The extent that the baseline varies between different cortical areas of interest in the cortex is not clear at this time, so there are reservations concerning the interpretation of the amplitude of the BOLD signal recorded in different areas during a psychological test.107
The existence of a ‘global BOLD signal’ across the cortex, and the question of the regression of the signal
In fMRI the ‘global signal’ refers to the average signal across all brain voxels in the time series of signal intensity. Given that most fMRI studies aim to specify changes in activity that are specific to particular brain areas, and that this global signal is at least in part generated by sources other than those due to such activity, such as the cardiac and respiratory cycles,108 it is important to identify such contributions and eliminate them from the global signal. The easiest path to take here involves elimination of the entire global signal from measurements by simply regressing it from the time series of the signals in each voxel, using linear regression.109 An important caveat to this apparently straightforward procedure is that the mathematical techniques involved can introduce spurious relations between fMRI measurements in different brain voxels. Opposite conclusions have been reached by senior brain imaging experts concerning whether to regress out the global signal or not.110 This has left researchers in a quandary as to how to proceed, a quandary that has been highlighted by the contending groups in joint papers.111 Such difficulties are compounded by the possibility that the global signal might contain biological information concerning brain function, for which there is evidence.112 All in all, there is a need for caution when deliberating on fMRI observations claiming to delineate different functional networks of activity in the brain.
Notes
1 1 For a history of the identification of sensory systems see M. R. Bennett, S. Hatton, D. F. Hermens and J. Lagopoulos, ‘Behaviour, neuropsychology and fMRI’, Progress in Neurobiology, 145–6 (2016), pp. 1–25.
2 2 Aristotle, De Anima 412a20. Subsequent references to this treatise in the text will be flagged ‘DA’.
3 3 It should be noted that Aristotle held that a sightless eye is no more an eye than a painted eye, just as a corpse is no more an animal than a statue.
4 4 Note that when Aristotle says that we do these things with our soul, this is not like doing something with our hands or eyes, but rather like doing something with our talents and abilities.
5 5 We disregard here the complexities, and incoherences, that arise with regard to Aristotle’ s distinction between the active and passive intellect and the intimation that the active intellect may be capable of existing without a body (DA 429a18–29, 430a18–25). These passages were crucial for the later scholastic synthesis of Aristotelian philosophy of mind with Christian doctrine concerning the immortality of the soul.
6 6 For his reasoning, see De Partibus Animalium 647a22–34. In this respect he differed from the Hippocratic tradition. The Hippocratic lecture on epilepsy noted that ‘It ought to be generally known that the source of our pleasure, merriment, laughter and amusement, as of our grief, pain, anxiety and tears, is none other than the brain. It is especially the organ which enables us to think, see and hear, and to distinguish the ugly and the beautiful, the bad and the good, pleasant and unpleasant… . It is the brain too which is the seat of madness and delirium, of the fears and frights which assail us, often by night, but sometimes even by day; it is there where lies the cause of insomnia and sleepwalking’ (‘The sacred disease’, §17, in G. E. R. Lloyd (ed.), Hippocratic Writings (Penguin Books, Harmondsworth, 1978). The Hippocratic insight is wonderful; the physiological reasoning is, however, no less erroneous than Aristotle’ s reasoning in support of his different hypothesis.
7 7 Aristotle, De Somno 455a21. This is the Barnes translation; an alternative translation of the sentence in which this term appears is ‘For there exists a single sense-faculty, and the master organ is single’.
8 8 His term is aisthe¯sis koine¯, which occurs only in De Anima 425a27, De Memoria 450a10 and De Partibus Animalium 686a27.
9 9 Aristotle, De Sensu 449a5–11.
10 10 See, e.g., F. Crick, The Astonishing Hypothesis (Touchstone, London, 1995), p. 22; A. Damasio, The Feeling of What Happens (Heinemann, London, 1999), p. 320; E. Kandel and R. Wurtz, ‘Constructing the visual image’, in E. R. Kandel, J. H. Schwartz and T. M. Jessell (eds), Principles of Neural Science (Elsevier, New York, 2001), p. 492. For a discussion of the binding problem, see §5.2.3.
11 11 Aristotle, De Somno 455a15–20.
12 12 e.g. L. Weiskrantz, Blindsight: A Case Study and Implications (Oxford University Press, Oxford, 1986); Weiskrantz, ‘Varieties of residual experiences’, Quarterly Journal of Experimental Psychology, 32 (1980), pp. 365–86. See §17.3.1.
13 13 The argument is curious, inasmuch as it is unclear in what sense he thinks we discriminate white from sweet. To be sure, we possess the faculties to see white things and distinguish them from other coloured things, and to taste sweet things and distinguish them from things with other tastes, and we (language-users) also possess the concepts of white (and other colours) and sweet (and other gustatory qualities). But we do not discriminate white things from sweet things; nor do we need any further organ to differentiate white from sweet (for what would it be to confuse them?).
14 14 Aristotle, De Somno 455a.
15 15 Aristotle, De Memoria 450a9–14.
