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"Freeman, Alan"
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A model for the origin and development of visual orientation selectivity
Orientation selectivity is a key property of primary visual cortex that contributes, downstream, to object recognition. The origin of orientation selectivity, however, has been debated for decades. It is known that on- and off-centre subcortical pathways converge onto single neurons in primary visual cortex, and that the spatial offset between these pathways gives rise to orientation selectivity. On- and off-centre pathways are intermingled, however, so it is unclear how their inputs to cortex come to be spatially segregated. We here describe a model in which the segregation occurs through Hebbian strengthening and weakening of geniculocortical synapses during the development of the visual system. Our findings include the following. 1. Neighbouring on- and off-inputs to cortex largely cancelled each other at the start of development. At each receptive field location, the Hebbian process increased the strength of one input sign at the expense of the other sign, producing a spatial segregation of on- and off-inputs. 2. The resulting orientation selectivity was precise in that the bandwidths of the orientation tuning functions fell within empirical estimates. 3. The model produced maps of preferred orientation-complete with iso-orientation domains and pinwheels-similar to those found in real cortex. 4. These maps did not originate in cortical processes, but from clustering of off-centre subcortical pathways and the relative location of neighbouring on-centre clusters. We conclude that a model with intermingled on- and off-pathways shaped by Hebbian synaptic plasticity can explain both the origin and development of orientation selectivity.
Journal Article
A model for the development of binocular congruence in primary visual cortex
Neurons in primary visual cortex are selective for stimulus orientation, and a neuron’s preferred orientation changes little when the stimulus is switched from one eye to the other. It has recently been shown that monocular orientation preferences are uncorrelated before eye opening; how, then, do they become aligned during visual experience? We aimed to provide a model for this acquired congruence. Our model, which simulates the cat’s visual system, comprises multiple on-centre and off-centre channels from both eyes converging onto neurons in primary visual cortex; development proceeds in two phases via Hebbian plasticity in the geniculocortical synapse. First, cortical drive comes from waves of activity drifting across each retina. The result is orientation tuning that differs between the two eyes. The second phase begins with eye opening: at each visual field location, on-centre cortical inputs from one eye can cancel off-centre inputs from the other eye. Synaptic plasticity reduces the destructive interference by up-regulating inputs from one eye at the expense of its fellow, resulting in binocular congruence of orientation tuning. We also show that orthogonal orientation preferences at the end of the first phase result in ocular dominance, suggesting that ocular dominance is a by-product of binocular congruence.
Journal Article
A General Theory of Value and Money: Foundations of an Axiomatic Theory
This article is the first part of “A General Theory of Value and Money” and sets out the foundations of an axiomatic theory of value and money. Its purpose is to equip the public to explain the four facts that define the modern capitalist economy: the long-term post-war decline of the economies of the global North, the financial instability which produced the 2008 crash, the prolonged post-war growth in inequality between the global North and the global South, and the exceptional 30-year growth of China. The axiomatic method is poorly understood among economists. It allows us to interrogate empirical evidence in a systematic way, and is therefore the necessary precondition for an inductivist, scientific enquiry into economic events, that is, an enquiry whose starting point is the explanation of facts. The second part of “A General Theory of Value and Money,” to follow, deals with the empirical conclusions, especially the analysis of accumulation, the rate of profit, and financial crashes.
Journal Article
Acceptance Speech for World Marxian Economics Award (III) How The West Was Lost
I am honoured and grateful for this award. To distinguish the contribution of an individual to a collective endeavour is always a trial; When the individual is oneself, it is almost impossible. The intellectual spine of any rational understanding of world history and political economy today is the work of Karl Marx. As with any thinker, we can engage with Marx only if we strive to understand his actual ideas, not the ideas that other thinkers attribute to him. To this end I have worked collectively, for 30 years, to develop the Temporal Single System Interpretation (TSSI) of Marx and thereby reclaim Marx’s value theory.
Journal Article
Motions Add, Orientations Don’t, in the Human Visual System
Humans can distinguish between contours of similar orientation, and between directions of visual motion. There is consensus that both of these capabilities depend on selective activation of tuned neural channels. The bandwidths of these tuned channels are estimated here by modelling previously published empirical data. Human subjects were presented with a rapid stream of randomly oriented gratings, or randomly directed motions, and asked to respond when they saw a target stimulus. For the orientation task, subjects were less likely to respond when two preceding orientations were close to the target orientation but differed from each other, presumably due to a failure of summation. For the motion data, by contrast, subjects were more likely to respond when the vector sum of two previous directions was in the target direction. Fitting a cortical signal-processing model to these data showed that the direction bandwidth of motion sensors is about three times the bandwidth of orientation sensors, and that it is the large bandwidth that allows the summation of motion stimuli. The differing bandwidths of orientation and motion sensors presumably equip them for differing tasks, such as orientation discrimination and estimation of heading, respectively.
Journal Article
The profit rate in the presence of financial markets: A necessary correction
In the past two decades the number, variety, and monetary value of marketable financial instruments, particularly securitized instruments, has grown by orders of magnitude. This is the most significant development in what many writers, for the most part Marxist, term 'financialisation'. It brings to light, however, an anomaly in the way they calculate the profit rate. This calculation takes no account of the capital tied up in these instruments.
Journal Article
A Multi-Stage Model for Fundamental Functional Properties in Primary Visual Cortex
Many neurons in mammalian primary visual cortex have properties such as sharp tuning for contour orientation, strong selectivity for motion direction, and insensitivity to stimulus polarity, that are not shared with their sub-cortical counterparts. Successful models have been developed for a number of these properties but in one case, direction selectivity, there is no consensus about underlying mechanisms. We here define a model that accounts for many of the empirical observations concerning direction selectivity. The model describes a single column of cat primary visual cortex and comprises a series of processing stages. Each neuron in the first cortical stage receives input from a small number of on-centre and off-centre relay cells in the lateral geniculate nucleus. Consistent with recent physiological evidence, the off-centre inputs to cortex precede the on-centre inputs by a small (∼4 ms) interval, and it is this difference that confers direction selectivity on model neurons. We show that the resulting model successfully matches the following empirical data: the proportion of cells that are direction selective; tilted spatiotemporal receptive fields; phase advance in the response to a stationary contrast-reversing grating stepped across the receptive field. The model also accounts for several other fundamental properties. Receptive fields have elongated subregions, orientation selectivity is strong, and the distribution of orientation tuning bandwidth across neurons is similar to that seen in the laboratory. Finally, neurons in the first stage have properties corresponding to simple cells, and more complex-like cells emerge in later stages. The results therefore show that a simple feed-forward model can account for a number of the fundamental properties of primary visual cortex.
Journal Article
The Poverty of Statistics and the Statistics of Poverty
This paper offers a critique of the picture of world growth and world inequality generally disseminated by international agencies. The positive view commonly presented depends on the widespread consensus that economic performance should be measured using 'Purchasing Power Parity' (PPP) statistics, instead of market exchange rates. Although originally conceived narrowly as a basis for comparing living standards, PPP indicators are now indiscriminately promoted as an unexceptionable standard for comparing and aggregating national income statistics. This article highlights the flaws in the PPP approach by accepting the claims made on their behalf at face value. It shows that, even on the basis of these claims, the wrong conclusions have been drawn. By comparing PPP and market exchange rate measures of inequality it shows that what really took place, at the end of the last century, was a systematic reduction in the prices of consumption goods in the Third World. PPP statistics have concealed this underlying and unsustainable trend, allowing it to be packaged as a stable reduction in poverty. Neither genuine growth, nor lasting poverty reduction was achieved over this period. The fall in the price of consumer goods masked a systematic failure to overcome the central problem of development-the high price of capital goods, which PPP statistics understate, and of intermediate goods, which they completely omit.
Journal Article
Pilot Single-Blind Trial of AbobotulinumtoxinA in Oromandibular Dystonia
Oromandibular dystonia (OMD) causes involuntary movements of masticatory and lingual muscles impairing eating, speaking, and swallowing. Treatment options are limited. The objective of this study was to determine the safety and efficacy of abobotulinumtoxinA (aboBoNTA) in OMD. A dose-finding study (phase 1) followed by a single session, prospective, single-blind trial (phase 2) was carried out. OMD subjects were evaluated at baseline, 6 and 12 weeks. Muscles injected were tailored to individual symptoms using EMG guidance, but the aboBoNTA dose for each muscle was pre-specified based on phase 1 results. Evaluations were Global Dystonia Rating Scale (GDS), Unified Dystonia Rating Scale (UDRS), Clinical Global Impression (CGI) improvement and severity, and quality of life (OMDQ-25). Adverse events were monitored. The lowest dosage in phase 1 resulted in adverse effects in two of three patients and thus was used in phase 2. In phase 2, adverse effects were observed in 50% of subjects including dysphagia, voice change, and soft palate weakness. Most were mild. Significant improvement was seen in quality of life (OMDQ-25), speech (BFMq21), and change in GDS, UDRS, CGI severity assessed by the unblinded investigator, but not in blinded video ratings. We conclude that aboBoNTA therapy in this study was associated with improved quality of life and was generally well tolerated in OMD, but occurrence of dysphagia dictated the importance of using low genioglossus dosing. Face to face assessment appears to be more sensitive than video assessment for change in OMD severity. Consideration of the disability in OMD places constraints on traditional placebo-control trial design. Development of novel trial designs is warranted.
Journal Article