The OnTheHuman site has an article by J. Prinz (here). I certainly like his approach and find his arguments very convincing.
We … ask which of our psychological states can be conscious. Answers to this question range from boney to bulgy. At one extreme, there are those who say consciousness is limited to sensations; in the case of vision, that would mean we consciously experience sensory features such as shapes, colors, and motion, but nothing else. This is called conservatism (Bayne), exclusivism (Siewert), or restrictivism (Prinz). On the other extreme, there are those who say that cognitive states, such as concepts and thoughts, can be consciously experienced, and that such experiences cannot be reduced to associated sensory qualities; there is cognitive phenomenology. This is called liberalism, inclusivism, or expansionism. If defenders of these bulgy theories are right, we might expect to find neural correlates of consciousness in the most advanced parts of our brain. …
Not only do I think consciousness is restricted to the senses; I think it arises at a relatively early level of sensory processing. Consider vision. According to mainstream models in neuroscience, vision is hierarchically organized. Lets consider where in that hierarchy consciousness arises. I think consciousness arises at the intermediate level. We experience the world as a collection of bounded objects from a particular point of view, not as disconnected, edged, or viewpoint invariant abstractions. I think this is true in other senses as well. For example, when we listen to a sentence, the words and phrases bind together as coherent wholes (unlike low-level hearing), and we retain specific information such as accent, pitch, gender, and volume (unlike high-level hearing). Across the senses, the intermediate-level is the only level at which perception is conscious. …
Expansionists say we can be conscious of concepts and thoughts, and that such experiences outstrip anything going on at the intermediate-level of perception. Associative visual agnosia … cannot recognize objects, but they seem to see them. When presented with an object, they can accurately describe or even draw its shape, but they cant say what it is. Bayne thinks their experiences are incomplete. He thinks knowing the identity of an object changes our experience of it. This is intuitively plausible. Instead, we can suppose that our top-down knowledge of the meaning changes how we parse the image. imaginatively impose a new orientation; we segment figure and ground; and we generate emotions and verbal labels, which we experience consciously along with the image; these are just further sensory statesbodily feelings in the case of emotions, and auditory images in the case of words. I think features of this kind can also explain what is missing in agnosia. Without meaning, images can be hard to parse, and associated images and behaviors do not come to mind.
Another argument comes from Charles Siewert. He focuses on our experience of language. Sometimes, when hearing sentences, we undergo a change in phenomenology, and that change occurs as a result of a change in our cognitive interpretation of the meanings of the words. Phenomenology also changes when we repeat a word until it becomes meaningless, or when we learn the meaning of a word in a foreign language. In all these cases, we experience the same words across two different conditions, but our experience shifts, suggesting that assignment of meaning is adding something above and beyond the sound of the words. But there are many sensory changes that take place as a result of sentence comprehension. First, we form sensory imagery. Second, comprehension effects parsing. Third, comprehension entails knowing how to go on in a conversation Fourth, meaning effect emotions.
The third argument I will consider comes from David Pitt. He begins with the observation that we often know what we are thinking, and we can distinguish one thought from another. This knowledge seems to be immediate, not inferential, which suggests we know what we are thinking by directly experiencing the cognitive phenomenology of our thoughts. The most obvious reply is that knowledge of what we are thinking is based on verbal imagery. I think this is a kind of illusion. We erroneously believe that we are directly aware of the contents of our thoughts when we hear sentences in the minds ear. This belief stems from two things. First, we often use verbal imagery as a vehicle for thinking Second, when contemplating a word that we understand, we can effortlessly call up related words or imagery, which gives us the impression that we have a direct apprehension of the meaning of that word. Our fluency makes us mistake awareness of a word for awareness of what it represents.
Putting these points together, I think restrictivsts should admit that thinking has an impact on phenomenology, but that impact can be captured by appeal to sensory imagery including images of words, emotions, and visual images of what our thoughts represent. Expansionists must find a case where cognition has an impact on experience, without causing a concomitant change in our sensory states. Thats a tall order.
At this point the dispute between restrictivists and expansionists often collapses into a clash on introspective intuitions. By way of conclusion, I will try to break this stalemate by sketching five reasons for thinking restrictivism is preferable even if introspection does not settle the debate.
Next comes the arguments for excluding cognitive phenomenology.
To make a convincing case for cognitive phenomenology, expansionists should find a case where the only difference between two phenomenologically distinct cases is a cognitive difference. But so far, no clear, uncontroversial case has been identified.
The second argument points to the fact that alleged cognitive qualities differ profoundly from sensory qualities in that the latter can be isolated in imagination. If other qualia can be isolated, why not cognitive qualia?
Third, it is nearly axiomatic in psychology that we have poor access to cognitive processes. The only processes we ever seem to experience consciously are those that we have translated, with great distortion, into verbal narratives.
A fourth argument follows on this one. The incessant use of inner speech is puzzling if we have conscious access to our thoughts. Why bother putting all this into words when thinking to ourselves without any plans for communication?
Finally, expansionism seems to dash hopes for a unified theory of consciousness. But there is little reason to think a single mechanism could explain how both perception and thought can be conscious, if cognitive phenomenology is not reducible to perception. This is especially clear if the mechanism is attention. There is no empirical evidence for the view that we can attend to our thoughts. There are no clear cognitive analogues of pop-out, cuing, resolution enhancement, fading, multi-object monitoring, or inhibition of return. Thoughts can direct attention, but we cant attend to them. Or rather, thoughts become objects of attention only when they are converted into images, words, and emotions. Expansionists might say that thought and sensations attain consciousness in different ways, but, if so, why think that the term consciousness has the same meaning when talking about thoughts, if it does not refer to the same mechanism?
This fits with the idea that only what enters the cortex through the thalamus, can be involved in the thalamo-cortical loops that synchronize their firing during the conscious experience. This category is sensory input (except the bulk of smell) and input about movement and emotion input via the basal ganglia.