How substantial is the difference between the neural signal associated with seeing an image and the imagination of that image? Surely, it can not entirely copy the pathway from the sensory organs to the emotional centers, but approximately how large is the common pathway for both processes?
Is there some principal difference in the neural pathway of these two processes, or is the imagined image just more of a slurred version of the visual image?
Short answer
Visual perception and visual imagery draw on much of the same neural machinery.
Background
I have interpreted your question as: What are the common neural circuitries between visual sensation and the imagination of sensation?
In neuroimaging, mental imagery of visual images is a big deal. For example, there is a large body of literature on cross-modal activation of visual areas in the brain in blind people. It is known that visual deprivation results in neural plasticity and the recruitment of visual areas in the brain for other sensory systems. For example, blind Braille readers show activation of the primary visual cortex when reading Braille (reviewed in Stronks et al., 2015). However, interpretation of these findings is difficult in late-blind individuals, because they have experienced visual input earlier in life. Hence, while Braille reading they can be mentally reproducing the visual representation of the Braille cells using visual neural circuitry.
Indeed, it has been shown with fMRI that visual imagery and visual perception draw on most of the same neural machinery (Ganis et al., 2004). However, the spatial overlap of the activated regions is neither complete nor uniform. The overlap in this study was more pronounced in frontal and parietal regions than in temporal and occipital regions, indicating that cognitive control processes function comparably in both imagery and perception, but not identically.
Various studies reveal different results, however. In another imaging study, 'just' two-thirds of brain regions overlapped in visual sensation and imagery (Kosslyn et al., 1997). Indeed, the experimental task used may have important effects on study outcomes.
Most notably in this regard is that approximately half of the studies done have found activation of the primary visual cortex during imagery (Kosslyn et al., 1999). This is interesting, because the primary visual cortex is generally thought to be an early, low-level area in the visual system that depends on thalamic input that relays information from the optic nerve to the brain. I.e., it is generally believed to depend on sensory stimulation. Generally, only the higher-level associative visual areas are associated with higher cognitive processes.
References
- Ganis et al., Cognitive Brain Res (2004); 20: 226–41
- Kosslyn et al., Neuriomage (1997); 6; 320–34
- Kosslyn et al., Science (1999); 284; 167–70
- Stronks et al. Brain Res (2015); 1624: 140–52
I read some articles on the thalamus, can't currently remember which ones though, that suggested that there is overlaps, but when you are imagining something the thalamus blocks the signals from entering the perceptual system. This is still speculative though, as current the resolution of current measurement methods leaves a lot to be desired.
In some mental disorders the signal isn't blocked by the thalamus (if this is what happens) and is instead experienced as an hallucination, there is also some evidence that this also happens in a less intrusive way in some people we perhaps wouldn't not call mentally ill.
Nikola Tesla for example suffered from what we would call hallucinations, but was able to learn to control it to some extent so he could use them as a tool for helping him making machines. I think the following section of his biography is quite interesting.
There was another and still more important reason for my late awakening. In my boyhood I suffered from a peculiar affliction due to the appearance of images, often accompanied by strong flashes of light, which marred the sight of real objects and interfered with my thought and action. They were pictures of things and scenes which I had really seen, never of those I imagined. When a word was spoken to me the image of the object it designated would present itself vividly to my vision and sometimes I was quite unable to distinguish whether what I saw was tangible or not. This caused me great discomfort and anxiety. None of the students of psychology or physiology whom I have consulted could ever explain satisfactorily these phenomena. They seem to have been unique although I was probably predisposed as I know that my brother experienced a similar trouble. The theory I have formulated is that the images were the result of a reflex action from the brain on the retina under great excitation. They certainly were not hallucinations such as are produced in diseased and anguished minds, for in other respects I was normal and composed. To give an idea of my distress, suppose that I had witnessed a funeral or some such nerve-racking spectacle. Then, inevitably, in the stillness of night, a vivid picture of the scene would thrust itself before my eyes and persist despite all my efforts to banish it. Sometimes it would even remain fixed in space though I pushed my hand through it. If my explanation is correct, it should be able to project on a screen the image of any object one conceives and make it visible. Such an advance would revolutionize all human relations. I am convinced that this wonder can and will be accomplished in time to come; I may add that I have devoted much thought to the solution of the problem.
The rest of his autobiography has more details
