The Integration of Features in Visual Awareness : The Binding Problem By Andrew Laguna, S.J.
Outline I. Introduction II. The Visual System III. What is the Binding Problem? IV. Possible Theoretical Solutions to the Binding Problem V. The Temporal Correlation Hypothesis (TCH) VI. The Scholarly Debate on TCH VII. Evaluation of TCH and Current/Future Research
I. Introduction What is the biological basis of consciousness? Start by looking at conscious visual awareness
The ScientiOic Question What is the neural correlate of visual awareness? (Crick 1994, 204)
How do we deoine visual awareness? Vivid representation in our brains of the scene directly before us (Crick 1994, 207) Damasio: The problem of how the movie- in- brain is generated (Blackmore 2004, 231)
The Issue There is no single region of the brain that corresponds exactly to the vivid picture of the world we see in front of our eyes (Crick 1994, 159) Parallel processing in the visual system
II. The Visual System Review Light strikes the photoreceptors in the retina The optic nerve carries this information to the Lateral Geniculate Nucleus (LGN) of the thalamus Projection from the LGN to the occipital or striate cortex, also known as V1
In V1 Low- level visual processing occurs Retinotopic map of visual Oield Processing of edges, lines, and other basic features of the object
After V1 Processing continues via the temporal and parietal streams
Two Parallel Processes What System Ventral Runs from occipital lobe to inferior temporal lobe Features such as color, shape, and texture Object Properties Encoding Where System Dorsal Runs from occipital lobe to the parietal lobes Location, size, and orientation of the object Spatial Properties Encoding
Process of Visual IdentiOication
III. What is the Binding Problem? If the attributes of an object. are processed in distinct pathways, regions or modules, then how does the visual system bind these features color, shape and motion consistently and accurately into a single unioied precept? (Whitney 2009, 251)
The Binding Problem is a series of related problems Binding of features within an object to distinguish it from other objects Single feature construction Binding between sensory modalities Sensory- motor integration
Illustration of the Binding Problem
What the Binding Problem is not We are not concerned with associations that are learned over time.dynamic binding occurring in real time (Blackmore, 2004: 245)
IV. Possible Solutions to BP Convergent Hierarchical Coding Similar idea to grandmother cell theory A small group of neurons or a single neuron receives input from populations of neurons at lower levels in the hierarchy These cells respond to a particular combination of features For example, a higher- level cell would react only to an object of a particular shape and color at a particular retinal position
The Combinatorial Problem If there needs to be a cell for every possible pattern, it would quickly lead to a combinatorial explosion of the number of needed cells Brain can t support this
Another Solution: Population Coding A combination of features (i.e., an object) represented by a population of neurons distributed within and across the cortical hierarchy Each population has a distinct pattern of Oiring
The Superposition Problem What if two objects are presented in the same scene? Two populations of neurons would Oire, and they may be close to each other or overlapping If features and objects are represented solely by the Oiring of distributed populations of cells, how are the vast numbers of active neurons in the cortical network disambiguated from one another? (Gray 1999, 36)
V. The Temporal Correlation Hypothesis (TCH) Formulated independently by Legendy in 1970, Milner in 1974, and von der Malsburg in 1981 Also known as neuronal synchrony or synchronization of neuronal Oiring Overcomes the superposition problem: Different neuronal populations are distinguished from one another by the timing of spikes relative to neuronal oscillations
What is neuronal synchrony?
How is TCH empirically shown? Experiments must be designed that rely on the measurement of neuronal activity at more than one site simultaneously Insert electrodes into neurons of living Macaque monkey and cat The cells should have overlapping receptive Oields and represent elements that are normally bound together The animal performs a task to indicate whether correct binding occurs
Mathematical Analysis Create a Poststimulus Time Histogram (PSTH) of the activity recorded from each of the electrodes Do a cross- correlation, which measures the similarity of the Oiring oscillations
VI. The Scholarly Debate over TCH First received great attention with Crick and Koch s 1990 paper, Toward a Neurobiological Theory of Consciousness Suggested that synchronized Oiring on, or near, the beat of a gamma oscillation (in the 35- to 75- Hertz range) might be the neural correlate of visual awareness (Crick 1994, 245)
Supporting evidence for TCH We can say for certain that it does occur in the visual system, especially in V1 Firing rate occurs on a time range of 1-10ms Oscillatory modulation of cell Oiring has been discovered in a frequency range between 30 Hz and 90 Hz
Critique of TCH A synchronization- based mechanism is physiologically implausible in nerve cells (Shadlen and Movshon) How can a neuron engage in selective synchronous interactions with a subset of its inputs when a large percentage of all the cell s inputs are active and synchronous?
Gray s Response Gray s response: There are cellular mechanisms that allows neurons to be sensitive to the timing of their synaptic inputs Inhibition can alter neuronal activity on a millisecond time scale Intrinsic voltage- gated conductances Lead to currents with voltage- and time- dependent properties These currents can act postsynaptically to amplify the response of neurons to synchronous synaptic inputs
Other Problems DifOicult to replicate experimental Oindings Some experiments either failed to Oind oscillatory activity in the primate visual system Or this activity was not stimulus dependent Also, Oindings are interpreted differently among scientists Some argue that temporal correlation is not necessary for binding
Important Questions The hypothesis is not a theory about how binding is computed; it is a theory about how binding is signaled This begs the question of what algorithms are actually used to solve the binding problem how does the visual system decide which elements are part of single objects and which belong to different objects? (Shadlen and Movshon 1999, 68) How is the output of the binding process interpreted and used by later stages? (Treisman 1999, 110)
VII. Evaluation of TCH and Current/Future Research Role of neuronal synchrony uncertain today Now it s thought to be too slow to account for normal object perception in familiar situations Neural synchrony is perhaps involved in recognition learning, which has a longer time course May also be involved in attention Spikes from sending neurons driven by attended stimuli are more precisely gamma- band synchronized than spikes driven by unattended stimuli. (Velik 2010, 999)
Conclusion Neural synchrony is not the sole solution to the Binding Problem, let alone the neural correlate of visual awareness Perhaps it is part of a larger framework Top- down processes are probably the main players in this framework E.g., attention, knowledge, memory, and expectation The study of binding is in its infancy. (Velik 2010, 995)
Getting Closer to a Solution Recent study combined a novel visual stimulus with recent developments in the analysis of functional magnetic resonance imaging (fmri) data to show that the features of a pattern, such as color and motion direction, are conjointly represented (bound) even at the earliest stages of cortical visual processing. (Whitney 2009, 251)
References Blackmore, Susan. 2004. Consciousness: An Introduction. New York: Oxford University Press. Crick, Francis. 1994. The Astonishing Hypothesis: The Scientific Search for the Soul. New York: Charles Scribner s Sons. Crick, Francis and Christof Koch. 1990. Towards a Neurobiological Theory of Consciousness. Seminars in the Neurosciences 2 (1990): 263-275. Gray, Charles M. 1999. The Temporal Correlation Hypothesis of Visual Feature Integration: Still Alive and Well. Neuron 24 (September): 31-47. Koch, Christof. 2004. The Quest for Consciousness: A Neurobiological Approach. Englewood: Roberts and Company Publishers.
References Kosslyn, Stephen M. and Olivier Koenig. 1992. Wet Mind: The New Cognitive Neuroscience. New York: The Free Press. Shadlen, Michael N. and J. Anthony Movshon. 1999. Synchrony Unbound: A Critical Evaluation of the Temporal Binding Hypothesis. Neuron 24 (September): 67-77. Treisman, Anne. 1999. Solutions to the Binding Problem: Progress through Controversy and Convergence. Neuron 24 (September): 105-110. Velik, Rosemarie. 2010. From Single Neuron-firing to Consciousness Towards the True Solution of the Binding Problem. Neuroscience and Biobehavioral Reviews 34: 993-1001. Whitney, David. 2009. Neuroscience: Toward Unbinding the Binding Problem. Current Biology 19 (6): 251-253.