On sense perception:

Sight takes place by means of a physical impression...which does not occur spontaneously in the pupil of the eye. Instead, the air in between the eye and the object is compressed and stamped by both the object and the viewer, since [atoms] are always flowing from everything. (Theophrastus, 370-288 BCE)

They associated vision with certain images, identical in shape with the object, that constantly streamed from the object and impressed themselves on the eye. (Alexander of Aphrodisias, 3rd century CE)

On tastes:

He defines "sweet" as something [made of atoms that are] round and moderately large; "sour," as what is large, round, polygonal, and linear; whatever is "sharp tasting," as the name implies, sharp edges, and is angular, crooked, and linear; whatever is "pungent" is round, small, angular, and crooked; "salty" is angular, moderately large and crooked, with sides of equal length; whatever is "bitter" is round, small, crooked, and smooth; and "oily" is fine, round, and smooth. (Theophrastus, 370-288 BCE)

On knowledge:

There are two kinds of knowledge: one authentic, one illegitimate. The following belong to illegitimate knowledge: vision, hearing, smell, taste, and touch. The other [sc. rationality] is genuine and distinct from this kind... We must acknowledge that, for this reason, we are separated from reality... In fact, we know nothing about anything... (Sextus Empiricus, 2nd century CE)

Other fragments:

(1) According to the theory of Democritus it is the nature of the eternal objects [i.e. the atoms, out of which everything else comes into being] to be tiny substances infinite in number. Accordingly, he also postulates a place for them that is infinite in magnitude, which he designates by these names—the "void", the "nothing", the "infinite"; whereas he speaks of each individual atom as the "yes-thing", the "dense", and "being". He imagines them to be so small as to elude our senses, but as having all sorts of forms, shapes and different sizes. Treating these as elements, he imagines them as combining to produce visible and otherwise perceptible objects. As they move about in the void the atoms are at variance with each other because of their dissimilarity.... Thus in their movement, as they bump or even brush against each other, they tend to get entangled and interlocked.... When the substances [formed by interlocking atoms] remain joined for some length of time, it is explained by saying that they fit snugly and so catch hold of one another—for some atoms are scalene while others are sharply hooked, some are concave, others convex, and there are numerous other differences. [Aristotle]

(2) Some have identified soul with fire on the ground that fire is the element that is made up of the finest particles, being most nearly incorporeal, and also that it is preeminently the element that is in motion and that sets other things in motion. Democritus...regards soul and mind as the same thing, and he holds that this consists of primary indivisible bodies [i.e. atoms] that because of their fine shape and smoothness cause motion. Since the shape most susceptible of motion is spherical, it is of this shape that the atoms of mind and of fire consist. [ibid.]

(3) Democritus says that it is determined in the womb whether the offspring is to be male or female. However, he denies the theory [of Empedokles] that heat and cold are what make the difference; instead, he thinks it depends upon which of the two parents' generative fluids prevails. [ibid.]

(4) Differences of heaviness and lightness are explained by Democritus as ultimately differences of size.... He speaks in similar terms of hard and soft. According to his theory a thing is hard when its parts are compact, soft when they are loose, and differences of degree can be explained proportionately. The reason why differences of hard and soft are not always commensurate with differences of heavy and light is that they are not produced by the same positions and groupings of empty spaces. Thus although iron is harder than lead, lead is heavier than iron. This is because the iron is of uneven composition, and its empty spaces are more numerous and bigger although the particles are more condensed in some places than in others. But its average of empty spaces exceeds that of lead... He claims that whatever is sour is composed of atoms that are angular, tiny, thin and twisted. By its sharpness it slips in and penetrates everywhere, by its angular roughness it draws the parts of the tongue together and binds them.... But sweet consists of atomic figures that are rounded and not too hard; it softens the body by its gentle action. [Theophrastus]

(5) Vision is explained by Democritus in terms of optical reflection.... As a result of the streams of atoms that are always being transmitted by things, the air between the object and the eye is compressed and thus receives an imprint. The imprinted air, because it is now more solid and is of a color-hue that contrasts with that of the eye, is then reflected in the moisture that exists within both eyes. [ibid.]

(6) ...sound is a stream of atoms emitted by whatever...produces a noise of any kind. This stream is shattered into particles of similar shape—i.e. round going with round and irregular and triangular with their like. When they enter the ears they cause perception of the sound.... Democritus says that the air is also shattered into particles of similar shape and rolled along with the sound. [Aetius]